CI: Install PyQt5 and NumPy on Python 3.10

[hugovk]

Wheel now works on Python 3.10 so we can install PyQt5 there.

We can also install wheel early on in the CI. This avoids legacy 'setup.py install' for packages that only have sdists and no wheels, and then stores the wheel in the cache for the next run.

Using legacy 'setup.py install' for cffi, since package 'wheel' is not installed.
...
Using legacy 'setup.py install' for olefile, since package 'wheel' is not installed.

The cache isn't there for new branches, but will help master and updates to feature branches.

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NumPy is also buildable for 3.10 now, so we could remove this:

# TODO Remove condition when numpy supports 3.10
if ! [ "$GHA_PYTHON_VERSION" == "3.10-dev" ]; then python3 -m pip install numpy ; fi

But it takes about 5-6 minutes to build from sdist (3.10 only, others have wheels). It will be stored in the wheel cache for subsequent builds.

As we're now also aiming at getting 3.10 RC wheels out (#5569), shall we take the hit and also test NumPy on 3.10? It's possible we won't notice it given the large build matrix with many runners.

If not, hopefully there'll be NumPy wheels for 3.10 soon: numpy/numpy#19630