MTN drop support for python 3.7

[tomMoral]

As scikit-learn dropped support for python3.7, we can do the same for loky.

[ogrisel]

We also need to update the pypy CI to a 3.8+ compatible version of PyPy:

https://dev.azure.com/joblib/loky/_build/results?buildId=3335&view=logs&j=d4de2018-e8e4-5838-274d-7fdb37c3a433&t=c28e1389-c1d1-5d60-4928-1f8046c5e6ef&l=16

[ogrisel]

The PyPy test first fail with:

ERROR at teardown of TestExecutorDeadLock.test_crash_races[13]

where there are many more children processes at teardown than expected. This happens with other tests and then the PyPy CI job times out after 1h.

So there seems to be a problem related to terminating worker processes with recent versions of PyPy.