Use Wheels to make the tests run much much faster

Speed up installation into the testing virtual environment by creating Wheels for pure python distributions that don't already have Wheels on PyPI. Anything that requires a C compiler will still get compiled, so all these wheels are still cross platform.

#### Before:
```console
$ time tox
tox  575.18s user 28.32s system 94% cpu 10:35.57 total
```

#### After:
```console
$ time tox
tox  111.03s user 15.49s system 82% cpu 2:32.49 total
```