Prefer native strings on Python 2 when reading config files.

improve encoding handling for `setup.cfg`

tests: temporary pin pip to fix the Python 2.7 job

Given that the config file parsing functionality is unlikely to change upstream, just incorporate the functionality directly.

Pin pytest to <4 for now. Ref #1638.

In build_meta (PEP 517), always built gztar

Refactor build_meta

Always specify formats=gztar, overriding the project's legacy expectation that a zip sdist should be generated. Fixes #1623.

Tweak "making a pull request" documentation

reStructuredText is how the name is commonly stylized (see Wikipedia),
also fixed a typo in the "deprecation" description.

Spelling fixes

Disallow files for license inputs