Starting with tox 3.2.0, the commands for installing/listing
dependencies changed from pip to python -m pip, which is problematic
as this will leave bits of the previous (from the virtualenv creation)
older version of setuptools installed in the site-packages directory
because from pip's point of view, the active setuptools version is the
one in the current directory (which cannot be uninstalled).

Fix #1453.

Rationale:
- tox should work the same as before
- coverage is too slow and disabled by default locally
- disable coverage on pypy and pypy3 (way too slow)
- add coverage info for pkg_resources
- make sure tests are ignored in coverage
- make sure all CI jobs are identifiable
- include coverage for xfail (too complicated otherwise)
- disable report on terminal

Some of these environment variables are necessary codecov to
detect that it is on travis/appveyor and thus doesn't need
an upload token.