Drop support for Python 3.7

[jeffwidman]

I couldn't find an existing issue tracking dropping support for Python 3.7 (which was EOL'd at the end of June).

To be clear, I have no "user" need for this, it's more about making the lives of maintainers easier.

FWIW, we will be dropping support in later this month (and going forward tentatively hope to start aligning to the upstream Python support lifecycle), so as part of that work I was auditing the Python package managers. pip-tools / poetry already dropped 3.7, so the overall packaging ecosystem is moving away from 3.7 pretty quickly.

[matteius]

@jeffwidman actually I have been thinking to not drop py 3.7 until we move off requirementslib, because we wanted to have a good path forward for python 3.7 and Depndabot being able to make use of the pipenv upgrade command. Plus this work fixes a number of other edge cases around how how handle extras: #5793

Its kind of some lessons learned from dropping 3.6 too soon; by ensuring we migrate the code base and fix any critical issues in a final release before saying good by to 3.7 will be helpful to folks trying to upgrade past 3.7