Remove Python version 3.7 to unblock tests

[VladimirFokow]

Currently, Python 3.7 in tests blocks some tests, so no pull requests are possible.

• Python v.3.7 has reached its end-of-life, and v.3.8 will in 2024. Does this repo have to still support these versions?

• Dropping support for them can ease the burden on maintainers: the deprecated dependencies currently block the tests for this repo - thus no pull requests are possible.

• Plus, the newer Python versions can be supported here (until 3.12).

For example, numpy supports only Python 3.9+ as of 1.26.0 .

Which Python version should be the min. supported here: 3.8 or 3.9?

[pfmoore]

This repo shouldn't be using anything version-specific as it's intended just as an example. I don't see any objection to updating the Python requirement to 3.8, but I'm -1 on trying to track supported Python versions as that's just ongoing maintenance work (and we don't actually "support" any particular Python versions, so it's not even accurate in any practical sense).