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)