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.

Also install platform specific requirements in `extras_require`.

do not strip empty sections in `extras_require`

Revert "fix `install_requires` handling of extras"

This reverts commit a3ec721e.

Remove dependency on virtualenv. Setuptools' tests don't require it except pytest-virtualenv, which declares it.

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.