Use comments to mark all Python version bump areas

[polm]

There's many places in the code where things have to be modified when a new Python version is released, and every year I (or anyone making a PR) misses a few. A comment like PYVER should be added to make it easy to find all of these.

To make it clear, these are easy to miss because there are different ways to write versions (3.14, c314, 3.10~3.14, etc.), and multiple programming languages or formats are involved (Python, github actions, toml).