See https://bugs.python.org/issue2751

This allows to run the testsuite on systems where `/usr/bin/pip`
is a specific Python version, but the tests are run on a different one.

For example in Fedora RPM, when the Python 3 tests are invoked,
they should use Python 3 pip, not Python 2 `/usr/bin/pip`.

Unlike other approaches, like using `pip2`/`pip3` which is currently
done in Fedora RPM (downstream patch), this way it Works Everywhere™
and the downstream patch can be dropped.

See https://src.fedoraproject.org/rpms/python-setuptools/blob/54eaa03a4dc97f93a5e4c92c55e580a4ab55a058/f/0001-Run-test-on-a-version-specific-pip.patch

Proofreading for the setup.cfg documentation

travis: make deploy stage conditional

fix `test_pip_upgrade_from_source` test on Python 2.6

Add badges to the top of README

* PYPI
* Travis CI
* AppVeyor CI
* List of currently supported Python versions

Fix Python 2.6 support

pkg_resources: do not call stat() and access()

Improve README file list handling and add Markdown to the current list

pkg_resources: improve `WorkingSet.resolve(replace_conflicting=True)`

workaround easy_install bug

Add new long_description_content_type kwarg

Sort file lists