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.