For runtime dependencies, install_requires should be used. For build
dependencies, a PEP 518-compliant `pyproject.toml` should be used.

Other dependencies can use extra requirements.

Document building Cython projects instead of Pyrex

Exclude .pyc in sdist output

Added minumum column in metadata table and version info for long_description_content_type

This also documents that the long_description_content_type keyword was
added in version 38.6.0.
Co-authored-by: Gene Matusovsky <gene_matusovsky@s5a.com>

This should stop the PyPI release stage if a `.pyc` exists in the
generated SOURCES.txt.

Mark Py 2/3-only tests as skip instead of xfail

* Update setup.cfg documentation to include data_files

Refs #1395

Fixes #1414

Officially support Python 3.7

Fixes https://github.com/pypa/setuptools/issues/1528.

Also reuse pre-defined py2_only and py3_only decorators where
appropriate.

Fix easy install warning #1525

Adds call to os.path.abspath() in pkg_resources.normalize_path() on Cygwin

Add data_files support in setup.cfg

This adds the `[options.data_files]` section to the existing setup.cfg
example.

In the test case, dist.data_files needs to be sorted because the
current implementation loads the configuration files as a dictionary
with arbitrary order on Python < 3.6.

Update CI builds

We don't need it and it generates a couple of error messages on AppVeyor:

  FIND: Parameter format not correct
  [...]
  Error running `find 'C:\projects\setuptools' -not - [...]

Particularly: update pip so PEP 518 support is available.

Rational: https://blog.travis-ci.com/2018-10-04-combining-linux-infrastructures

Deprecate Subversion download functionality. Ref #1502. Used UserWarning instead of DeprecationWarning so it's visible to users who might be relying on this functionality.