Support python 3.8 to 3.10

[dalcde]

The python 3.8 support is partly based on #14. All (but one; see below) tests pass in python 3.7 to python 3.10 (I haven't tested earlier version but I don't expect them to fail)

test_async_generator_1 fails in python 3.8, but I believe this is not related to cloudpickle-generators at all; the asyncgen_asgen function fails to convert the original async generator into a generator (even before the cloudpickle roundtrip). I haven't looked too deeply into this, but I think that should be addressed in a separate PR/issue.

Python 3.11 support would require a lot more work, as the internals of frame objects was reworked.