Crash when invalid python_requires indicated in setup.cfg

Honor declared ordering for Provides-Extra entries in package metadata.

launcher: Fix build with mingw-w64

Fix typo in CHANGES.rst

Fix some usage of deprecated `imp` module

This reverts commit 42bd00ad.

travis: update PyPy jobs to use more recent versions

tests: fix `test_pip_upgrade_from_source` on Python 3.4

Do not test pip's master on 3.4, as support for it has been dropped.

Perform a more thorough check of package_data structure

Ensure the dictionary values are lists/tuples of strings.

Fix #1459.

value=None raises TypeError

DistutilsSetupError: 2 must be a list of strings (got None)

Include the file path in get_metadata() UnicodeDecodeErrors.

Fix CI

Use license classifiers rather than the license field.

stop using deprecated HTMLParser.unescape