Extract of collections documentation:
https://docs.python.org/dev/library/collections.html

"Deprecated since version 3.3, will be removed in version 3.9: Moved Collections Abstract Base Classes to the collections.abc module. For backwards compatibility, they continue to be visible in this module through Python 3.8."

Attached PR removes these aliases.

pip is broken by this change, causing test_venv to fail: html5lib still uses collections to get ABC.

FAIL: test_with_pip (test.test_venv.EnsurePipTest)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/vstinner/prog/python/master/Lib/test/test_venv.py", line 407, in do_test_with_pip
    self.run_with_capture(venv.create, self.env_dir,
  File "/home/vstinner/prog/python/master/Lib/test/test_venv.py", line 69, in run_with_capture
    func(*args, **kwargs)
subprocess.CalledProcessError: Command '['/tmp/tmpkxga42lx/bin/python', '-Im', 'ensurepip', '--upgrade', '--default-pip']' returned non-zero exit status 1.

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/home/vstinner/prog/python/master/Lib/test/test_venv.py", line 466, in test_with_pip
    self.do_test_with_pip(False)
  File "/home/vstinner/prog/python/master/Lib/test/test_venv.py", line 415, in do_test_with_pip
    self.fail(msg.format(exc, details))
AssertionError: Command '['/tmp/tmpkxga42lx/bin/python', '-Im', 'ensurepip', '--upgrade', '--default-pip']' returned non-zero exit status 1.

**Subprocess Output**
/tmp/tmpwhk_ss_e/pip-19.0.3-py2.py3-none-any.whl/pip/_vendor/pyparsing.py:3068: SyntaxWarning: invalid escape sequence \w
/tmp/tmpwhk_ss_e/pip-19.0.3-py2.py3-none-any.whl/pip/_vendor/pyparsing.py:3068: SyntaxWarning: invalid escape sequence \w
Traceback (most recent call last):
  File "/home/vstinner/prog/python/master/Lib/runpy.py", line 192, in _run_module_as_main
    return _run_code(code, main_globals, None,
  File "/home/vstinner/prog/python/master/Lib/runpy.py", line 85, in _run_code
    exec(code, run_globals)
  File "/home/vstinner/prog/python/master/Lib/ensurepip/__main__.py", line 5, in <module>
    sys.exit(ensurepip._main())
  File "/home/vstinner/prog/python/master/Lib/ensurepip/__init__.py", line 198, in _main
    return _bootstrap(
  File "/home/vstinner/prog/python/master/Lib/ensurepip/__init__.py", line 117, in _bootstrap
    return _run_pip(args + [p[0] for p in _PROJECTS], additional_paths)
  File "/home/vstinner/prog/python/master/Lib/ensurepip/__init__.py", line 27, in _run_pip
    import pip._internal
  File "<frozen zipimport>", line 259, in load_module
  File "/tmp/tmpwhk_ss_e/pip-19.0.3-py2.py3-none-any.whl/pip/_internal/__init__.py", line 40, in <module>
  File "<frozen zipimport>", line 259, in load_module
  File "/tmp/tmpwhk_ss_e/pip-19.0.3-py2.py3-none-any.whl/pip/_internal/cli/autocompletion.py", line 8, in <module>
  File "<frozen zipimport>", line 259, in load_module
  File "/tmp/tmpwhk_ss_e/pip-19.0.3-py2.py3-none-any.whl/pip/_internal/cli/main_parser.py", line 12, in <module>
  File "<frozen zipimport>", line 259, in load_module
  File "/tmp/tmpwhk_ss_e/pip-19.0.3-py2.py3-none-any.whl/pip/_internal/commands/__init__.py", line 6, in <module>
  File "<frozen zipimport>", line 259, in load_module
  File "/tmp/tmpwhk_ss_e/pip-19.0.3-py2.py3-none-any.whl/pip/_internal/commands/completion.py", line 6, in <module>
  File "<frozen zipimport>", line 259, in load_module
  File "/tmp/tmpwhk_ss_e/pip-19.0.3-py2.py3-none-any.whl/pip/_internal/cli/base_command.py", line 25, in <module>
  File "<frozen zipimport>", line 259, in load_module
  File "/tmp/tmpwhk_ss_e/pip-19.0.3-py2.py3-none-any.whl/pip/_internal/index.py", line 14, in <module>
  File "<frozen zipimport>", line 259, in load_module
  File "/tmp/tmpwhk_ss_e/pip-19.0.3-py2.py3-none-any.whl/pip/_vendor/html5lib/__init__.py", line 25, in <module>
  File "<frozen zipimport>", line 259, in load_module
  File "/tmp/tmpwhk_ss_e/pip-19.0.3-py2.py3-none-any.whl/pip/_vendor/html5lib/html5parser.py", line 8, in <module>
  File "<frozen zipimport>", line 259, in load_module
  File "/tmp/tmpwhk_ss_e/pip-19.0.3-py2.py3-none-any.whl/pip/_vendor/html5lib/_tokenizer.py", line 16, in <module>
  File "<frozen zipimport>", line 259, in load_module
  File "/tmp/tmpwhk_ss_e/pip-19.0.3-py2.py3-none-any.whl/pip/_vendor/html5lib/_trie/__init__.py", line 3, in <module>
  File "<frozen zipimport>", line 259, in load_module
  File "/tmp/tmpwhk_ss_e/pip-19.0.3-py2.py3-none-any.whl/pip/_vendor/html5lib/_trie/py.py", line 6, in <module>
  File "<frozen zipimport>", line 259, in load_module
  File "/tmp/tmpwhk_ss_e/pip-19.0.3-py2.py3-none-any.whl/pip/_vendor/html5lib/_trie/_base.py", line 3, in <module>
ImportError: cannot import name 'Mapping' from 'collections' (/home/vstinner/prog/python/master/Lib/collections/__init__.py)

html5lib has already been fixed:
https://github.com/vstinner/html5lib-python/commit/4f9235752cea29c5a31721440578b430823a1e69

But latest html5lib release was version 1.0.1 at... Dec 7, 2017 :-(

Paul Ganssle requested a new html5lib release... to fix the collections warning :-)
https://github.com/html5lib/html5lib-python/issues/419

Oh. I wasn't aware of the previous attempt: https://github.com/python/cpython/pull/10596 there is a long discussion there. The blocker issue was also html5lib.

Right now (pip 19.1.1?), "./python -m venv env" still fails because of html5lib/_trie/py.py. There is still no activitiy on the request to get a new release:
https://github.com/html5lib/html5lib-python/issues/419

In CPython commit 5380def8269b24a8a3bc46396373a1dc91b1dd1a, bundled pip is updated to 19.2.1, and since pip 19.2 an html5lib patch is backported [1] to "prefer importing from collections.abc instead of collections" [2]. I believe https://github.com/python/cpython/pull/10596 can be revisited now.

[1] https://github.com/pypa/pip/commit/ef7ca1472c1fdd085cffb8183b7ce8abbe9e2800
[2] https://github.com/pypa/pip/commit/3d6bb3a29676f880e84014d98afcb5ac74a9844e

I think this can be closed since https://bugs.python.org/issue25988 has the PR merged.

New changeset c47c78b878ff617164b2b94ff711a6103e781753 by Hugo van Kemenade in branch 'master':
bpo-37324: Remove ABC aliases from collections (GH-23754)
https://github.com/python/cpython/commit/c47c78b878ff617164b2b94ff711a6103e781753