Add testing for python 3.9 and 3.10

[tddough98]

In #287, we drop support for Python 3.6, but did not test for the newer Python releases 3.9 and 3.10. Here we add them to our test matrix to make sure our codebase works for these versions for when we'll need to switch to them down the road.

However, pytest-pep8 is incompatible with Python 3.10. Python 3.10 requires at least pytest 6.2.4 per this issue and pytest-pep8 is incompatible with pytest 6, as seen in this error message

Direct construction of Pep8Item has been deprecated, please use Pep8Item.from_parent.
See https://docs.pytest.org/en/stable/deprecations.html#node-construction-changed-to-node-from-parent for more details.

As pytest-pep8 is not an actively mantained project and the last release was in 2014, I switched to flake8 and pytest-flake8. Flake8 checks for pep8 as well as pyflakes and circular complexity, keeping the style checks and adding some error checks.

In summary, this PR contains:

• additions to tests.yaml to test Python 3.9 and 3.10
• switches out pep8 checks for flake8 checks
• fixes issues highlighted by flake8

[willgraf]

Looks good, just a couple questions/comments.