Fix requires handling

Internally move requirements in `install_requires` that are using
extras to `extras_require` so those extras don't get stripped when
building wheels.

Allow requirements of the form `"extra": ["barbazquux; {marker}"]`
by internally converting them to `"extra:{marker}": ["barbazquux"]`.

Use pytest-virtualenv so the test can be run no Windows too.

Fix environment markers handling

Run build stages (pipelines) in Travis CI config

Also:
* enable pip cache
* split script into install+script steps

Fix a typo: compatibilty -> compatibility

Tests releated fixes and tweaks

[WIP] Add a test that setuptools can be installed in a clean environment

Setting LC_ALL/LC_CTYPE=C does not work since LANG=en_US.UTF-8.

In my environment, with:

- LANGUAGE=en_US:en_GB:en:C
- LC_ALL=en_US.UTF-8

Running the testsuite with:

- python3.6 -m pytest: is successful
- tox -e py36: fails

The later because LC_ALL is unset by tox, and LANGUAGE is not passed
through, so `locale.getpreferredencoding()` returns 'ANSI_X3.4-1968'.

So it's possible to run tox with something like:
`py.test -k test_unicode_filename_in_sdist`, and
not loose the default report arguments.