build_ext: always import _CONFIG_VARS from distutils.sysconfig instead of sysconfig

README and CHANGES were renamed. Update MANIFEST.in to include them once again in the source distributions. Fixes #575.

otherwise `distutils.sysconfig.customize_compiler` does not configure OSX compiler for -dynamiclib

See https://github.com/pypa/setuptools/issues/571

Fix setuptools url and curl wants --location to follow redirects

This change also removes:

    sys.meta_path = [importer for importer in sys.meta_path if
                     importer.__class__.__module__ != 'pkg_resources.extern']

from _unload_pkg_resources.

These lines were removed from the version at: https://bootstrap.pypa.io/ez_setup.py

I don't know why the code there and here differs.

password prompt on upload command

Update packaging to 16.7

This adds support for the deprecated python_implementation marker
which was an undocumented setuptools marker prior to 20.2 in
addition to the newer markers.

The upload command wasn't prompting for a password. If there was no
password specified in ~/.pypirc, the upload command,
    python setup.py sdist upload
would fail and raise a TypeError. The workaround for this was to use
    python setup.py sdist register upload
since the register command does prompt for a password.

This commit ensures that the upload command prompts for a password
if not given one by ~/.pypirc or the register command.