Drop support for Python 3.6

[vkbo]

Summary:

Python 3.6 reached end of life on December 23rd, 2021. This PR removes all mentions of Python 3.6 support and drops testing against a 3.6 environment. Since 3.6 is dropped, the couple of places were custom code was used to avoid using 3.7 features have been updated.

Related Issue(s):

Reviewer's Checklist:

• The header of all files contain a reference to the repository license
• The overall test coverage is increased or remains the same as before
• All tests are passing
• All flake8 checks are passing and the style guide is followed
• Documentation (as docstrings) is complete and understandable
• Only files that have been actively changed are committed