Add new long_description_content_type kwarg

Sort file lists

Test that specifying a `long_description_content_type` keyword arg to
the `setup` function results in writing a `Description-Content-Type`
line to the `PKG-INFO` file in the `<distribution>.egg-info` directory.

`Description-Content-Type` is described at
https://github.com/pypa/python-packaging-user-guide/pull/258

This is used to populate the new `Description-Content-Type` field.

`Description-Content-Type` is described at
https://github.com/pypa/python-packaging-user-guide/pull/258

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

Don't reuse `easy_install` command in `Distribution.fetch_build_egg` implementation.

Fix #196.

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`.