Drop Python 3.7, which has reached end-of-life

[Zac-HD]

• Remove it from CI to unblock e.g. Make pytest.warns() re-emit all non-matching warnings #10937
• Update our pre-commit hooks to enforce 3.8+
• Check what's new in Python 3.8 for other new features, e.g. f"{x=}", typing changes, importlib.metadata, positional-only arguments, etc.

Previously: #7808 (Python 3.5), #9437 (Python 3.6).