GitHub Action to run pytest

[cclauss]

Based on the feedback at https://github.com/richpl/PyBasic/pull/57#issuecomment-962672683, a GitHub Action that only runs pytest. https://docs.pytest.org

Test results: https://github.com/cclauss/PyBasic/actions

Fixes a failing doctest: FlowSignal(ftype=FlowSignal.RETURN).ftarget is None, not -1

$ pytest --doctest-modules .

============================= test session starts ==============================
platform linux -- Python 3.10.0, pytest-6.2.5, py-1.10.0, pluggy-1.0.0
rootdir: /home/runner/work/PyBasic/PyBasic
collected 2 items

flowsignal.py .                                                          [ 50%]
lexer.py .                                                               [100%]

============================== 2 passed in 0.07s ===============================