Remove vendored dependency on six and remove support for Python 3.7

[fpgmaas]

This PR:

• Removes the dependency on six.
• Drops support for Python 3.7.
• Updates some development dependencies related to pytest, so the unit tests can be run on newer versions of Python as well.
• Adds the .python_version to .gitgnore.

For removing the dependency on six, I simply replaced six.moves.zip_longest with itertools.zip_longest as suggested by @MrPowers. I however did not test if this changes existing functionality, I assume that is already covered by the unit tests? Those ran uccessfully.

See also https://github.com/MrPowers/chispa/issues/78