Environmental variables set in `setenv` also affect pip invocations.
`HOME` determines where pip is placing its cache.
Setting HOME to anything virtual env-specific means that
every virtual environment is created with its own cache.
This wastes time, data and disk space.
* Import things from `aqt.qt` not `PyQt5`/`PyQt6`
* When testing with Qt6, disable Anki's Qt5 compatibility mode
* Depend on `pytest-anki` that is also Qt version agnostic
The tests can be run now using `tox` against multiple Anki versions;
see instructions in `tox.ini`.
The tests depend on `pytest-anki` that had to be slightly modified
to remove the upper constraint on Anki version, as well as
to remove a few dependencies that are not essential to using it.
