Expand CI workflow

[TinyMarsh]

Description

This PR expands the CI system to run the tests in Python 3.8-3.9 as well as in all three platforms: Linux, Windows and MacOS.

Close #104

Type of change

• New feature (non-breaking change which adds functionality)
• Optimization (non-breaking, back-end change that speeds up the code)
• Bug fix (non-breaking change which fixes an issue)
• Breaking change (whatever its nature)

Key checklist

• All tests pass: $ python -m pytest
• The documentation builds and looks OK: $ python setup.py build_sphinx

Further checks

• Code is commented, particularly in hard-to-understand areas
• Tests added that prove fix is effective or that feature works

[dalonsoa]

@TinyMarsh , unit tests with Python 3.9 in Windows is failing. I'm re-running the CI, in case it was a flaky test, but if it indeed fails, then we need to open an issue about it and remove Python 3.9 from the list (or indicate somewhere that it works in Python 3.9 but only in MacOS and Linux).

[TinyMarsh]

@dalonsoa It looks like tests have now passed for Windows with Python 3.9.