Don't run tests in parallel on Travis. Running the test suite in parallel
on Travis does not speed it up, so drop it. It aslo crashes pypy runs.

Move sudo required and dist xenial to base.

Remove testing against Python nigthly. Testing against dev is enough.

Skip the deploy stage entirely if not a PR.

Bump pre-commit version.

Appveyor build is only required if we modified the source files or the
build config.

* Formatting, clarify phrasing

Pulling out this functionality in its own paragraph, spelling out the name of the character (dash) and adding an example makes this feature more visible.

Might help with #870

* rst is not markdown

* Fix formatting

* Parralelize and randomize test runs by default

By running tests in parallel we can singificantly speed up our CI and
local tests. We also randomize the order of the tests to make sure we
don't have any latent dependencies in between tests.

* Do not use the projects test file for test_env_spec

* Version and help commands should have their own directory

* Fix test_env_variables_added_to_pcall fails if PYTHONPATH is set

* Calling the console script helps should have its own tox env too

With the release of 3.1 looming around and handled by myself decided there's no better time to automate than now.

This involves:
- move from a devpi wheel/sdist creation to a Travis PyPi release trigger
- have a tox target to automatically create release branches (we can then create a PR from this) (use Python instead of bash)
- have a tox target to send the release notifications (use Python instead of bash).

- add Python 3.8 dev tests as failure allowed
- Python 3.7 tests are no longer allowed to fail