Revert to using a copy of splituser from Python 3.8.

Revert to using a copy of splituser from Python 3.8. Using urllib.parse.urlparse is clumsy and causes problems as reported in #1663 and #1668. Alternative to #1499 and fixes #1668.

Bugfix/1663 restore port

Ensure a specified port in package_index isn't lost in the parse/unparse of the URL when auth is present. Fixes #1663.

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.