Pip installable up to python 3.9

[c-martinez]

Tested for Pythons 3.6.15 | 3.7.16 | 3.8.16 | 3.9.16 as follows:

virtualenv py3X --python python3X
source py3X/bin/activate
pip install .
pip install -r requirements-test.txt
pytest tests/
deactivate

(where the X is the python version 6, 7, 8 or 9)

[c-martinez]

Addresses #386

[c-martinez]

As part of testing this, we should make sure Github Action for testing passes tests for all versions of Python included here (6 to 9)

[rapw3k]

Hi @c-martinez i was testing for python 3.9 and it build ok...

[c-martinez]

Great, thanks for testing @rapw3k , good to know it not only works on my computer :-)

[c-martinez]

It looks like it is building properly in GitHub actions for all versions.

I think we can merge this, unless you see any issues @albertmeronyo ?