The following code causes an assertion failure on Windows:
class BadEnv(dict):
    keys = None

import subprocess

import sys

subprocess.Popen([sys.executable, "-c", "pass"], env=BadEnv())


this is because getenvironment() (in Modules/_winapi.c) calls PyMapping_Values()
immediately after calling PyMapping_Keys().
calling PyMapping_Values() ultimately leads to calling _PyObject_FastCallDict(),
which does 'assert(!PyErr_Occurred());'.
thus, in case of an error in PyMapping_Keys(), the assertion fails.

New changeset 0b3a87ef54a0112b74e8a1d8c6f87d10db4239ab by Serhiy Storchaka (Oren Milman) in branch 'master':
bpo-31471: Fix assertion failure in subprocess.Popen() on Windows, in case env has a bad keys() method. (#3580)
https://github.com/python/cpython/commit/0b3a87ef54a0112b74e8a1d8c6f87d10db4239ab

Is this bug reproducible in 2.7?

New changeset f135f62cfd1529cbb9326e53728a22afd05b6bc3 by Serhiy Storchaka (Miss Islington (bot)) in branch '3.6':
[3.6] bpo-31471: Fix assertion failure in subprocess.Popen() on Windows, in case env has a bad keys() method. (GH-3580) (#3584)
https://github.com/python/cpython/commit/f135f62cfd1529cbb9326e53728a22afd05b6bc3

in 2.7 getenvironment() is in PC/_subprocess.c, and it also calls PyMapping_Values()
immediately after calling PyMapping_Keys().
however, _PyObject_FastCallDict() doesn't exist here.
in case of an error in both PyMapping_Keys() and PyMapping_Values(), the
error in PyMapping_Values() just overwrites the error in PyMapping_Keys().

but I haven't gone over all of the code that could be run as part of
PyMapping_Values(), so I am not sure whether something could go wrong in case PyMapping_Keys() failed.

Then I think it is worth to backport the fix to 2.7.

OK.
but there isn't an assertion failure to test in 2.7, so is adding a test
still relevant?

It's on you.

New changeset c7f165fe652651c32833245fc902c790a4f173fa by Serhiy Storchaka (Oren Milman) in branch '2.7':
[2.7] bpo-31471: Fix assertion failure in subprocess.Popen() on Windows, in case env has a bad keys() method. (GH-3580) (#3595)
https://github.com/python/cpython/commit/c7f165fe652651c32833245fc902c790a4f173fa

Thanks Oren!