MockAwareDocTestFinder has stale docstring

[jaraco]

The docstring for MockAwareDocTestFinder references issues that are closed.

pytest/src/_pytest/doctest.py

Lines 507 to 512 in 043ff9a

	class MockAwareDocTestFinder(doctest.DocTestFinder):
	"""A hackish doctest finder that overrides stdlib internals to fix a stdlib bug.
	https://github.com/pytest-dev/pytest/issues/3456
	https://bugs.python.org/issue25532
	"""

issue25532 resolves to python/cpython#69718 with the fix released in Python 3.6. #3456 is also closed.

This docstring should be updated to reflect its current purpose. If there's any compatibility code still relevant to those issues, that too should be removed.

[jaraco]

This issue can be addressed in #12432.