Tests: Run `python -m pip` instead of plain `pip`

Clean up resource warnings during tests

tests: make sure some essential tests are run

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

When tests are invoked with the Python flag -Wall, warnings appear in
the form of:

  ResourceWarning: unclosed file ...

Close all files and resources deterministically to avoid such warnings.
Most often, easiest to do using a context manager.

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