GitHub Action to run tox

[cclauss]

Because Travis CI is on vacation.

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

 >           assert r0.content == r1.content
E           assert b'{\n  "args"...org/get"\n}\n' == b'{\n  "args"...org/get"\n}\n'
E             At index 208 diff: b'3' != b'2'
E             Use -v to get more diff

tests/integration/test_record_modes.py:43: AssertionError

https://github.com/betamaxpy/betamax/blob/master/tests/integration/test_record_modes.py#L43