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

* Update setup.cfg documentation to include data_files

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.