New flake8 gives errors

[LourensVeen]

The newly released flake8 version 6 gives some spurious "X imported but unused" errors for some of our type annotations. This is because flake8 version 6 upgrades to pyflakes version 3, which removed support for them (see PyCQA/pyflakes#684).

Type comments were introduced in PEP-484, and were the only way to write type annotations for variables until Python 3.6, when PEP-526 was implemented. So instead of primes = [] # type: List[int] we now get to write primes: List[int] = [].

We're kind of in the midst of things at the moment, so the short term solution is to specify flake8<6 in tox.ini so the tests will run. Then we can go and remove all our type comments when the checkpointing/profiling/shared-mem dust settles down a little.