Allow combining several files in metadata.long_description

If one wishes to test both 'mixed' and 'sandboxed', let's create a separate test for that, but it's probably unnecessary.

to generate reproducible zip files
that do not differ depending on (random) filesystem order

See https://reproducible-builds.org/ for why this matters.

Now it's possible to use it like this:
* long_description = file: README.rst, CHANGELOG.rst, LICENSE.rst

* `file:` not accepts comma-separated list of filenames
* files' contents are glues with an LF separator

Rename [wheel] section to [bdist_wheel] as the former is legacy

Fix exception on mingw built Python 2

msvc9compiler doesn't like being imported on mingw built Python. It
throws DistutilsPlatformError, so catch it.

Fixes #1118

The exception is caused by the VCForPython27 monkey patch. It's not needed on mingw built Python. Disable it and avoid the import by using the same function that `distutils` uses to decide it's running on a mingw built Python.

Fixes #1118

fix requires handling when using setup.cfg

Better detect unpacked eggs

Do not assume a directory named in `.egg` is an egg, unless it has an
actual egg metadata directory.

Closes #462

But our tests do, one that provides `--no-wheel`.

Directory used by setuptools to auto-install dependencies

travis: fix deployment

update Travis builds

tests: disable pytest-flake8 on Python 2.6

Fix `test` command handling of `extras_require`

Python 2.6 support has been official dropped from
flake8 for some time, and broken since 3.4.0.