Drop support for Python 3.7

[mdickinson]

Python 3.7 is beyond end-of-life, and no longer supported. There are a couple of places where supporting Python 3.7 adds (fairly minor) complications to the current codebase. I'd suggest dropping support for Python 3.7 in the next feature release of Traits.

[mdickinson]

Of particular note:

• 1767 fixes build errors on Python 3.13 but introduces an ugly #ifdef in order to support Python 3.7.

• Python 3.7 raises deprecation warnings for containment checks of std. lib. Enum values in Enum collections, and we have special code to work around that (see safe_contains in traits/trait_base.py.

[mdickinson]

• Python 3.7 raises deprecation warnings for containment checks [...]

Sorry, that's bogus: we still need safe_contains to handle the TypeError that's raised from Python 3.8 through Python 3.11. Python 3.12 goes back to returning False (which IMO was the correct behaviour all along).

[mdickinson]

This was done in #1773.