Revert "Prefer new_build_ext for Cython builds as recommended by Cython project. Fixes #1270." Reopens #1270. Fixes #1271.

This reverts commit 038baa16.

Import Cython.Compiler.Main as recommended by Cython project to improve Cython detection. Fixes #1229.

Reflect find_packages behavior in doc

fix Python 3.7 support

Fix dry-run handling

- update scanning code to handle pyc header change
- handle change to `Exception.__repr__` output

find_packages doesnt find PEP420 packages but
the documentation suggests that it does.

see: pypa/setuptools#97

Removed warning when PYTHONDONTWRITEBYTECODE is enabled

Support PEP 345 Project-URL metadata

Add setup.cfg support for long_description_content_type

Fix trailing slash handling in pkg_resources.ZipProvider

Given a ZipProvider, if the underlying ZipImporter prefix is empty,
then resource_listdir('') and resource_listdir('subdir/') fail, while
resource_listdir('/') and resource_listdir('subdir') succeed.

On the other hand, if the underlying ZipImport prefix is not empty,
then resource_listdir('/') fails but resource_listdir('') succeeds.

With this change, the cases listed succeed with or without trailing
slashes.

Add an entry to the Setuptools usage documentation for the
project_urls dict, and include it in the "advanced" project example.

While at it, adjust the dogfooding use in Setuptools' own setup.py
to use HTTPS for the Documentation URL and drop redundant entries
for Bug Tracker and Source Code.

Also remove a no-op attribute reassignment from egg_info.py.