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.

Move Python 2.6 exception into specific test, capturing that the expected order is different on Python 2.6, not because of the lack of OrderedDict, but because of different behavior in RawConfigParser. Ref #553.