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.

Added .idea/ to gitignore, to cover cases where people are using a Jetbrains text editor

build_meta sdist directory delegate to --dist-dir

Revert "Make safe_name compliant to PEP 503 and behaviour of pip > 8.1.2"

Improved handling of  module __path__ attribute for namespace packages, fixes #1321

Make safe_name compliant to PEP 503 and behaviour of pip > 8.1.2