[eudsl-python-extras] drop support for generics for py < 3.12

[makslevental]

After much consternation and flaggelation, I've decided we can't/shouldn't support generics for python < 3.12. The context/reason is that I thought I had robust support for just plain TypeVars as globals (and in the closure) but recently I've realized that that support is not robust.

In the weeds: the "generic" kernel instantiation effectively mutates the TypeVar and thus when the TypeVar is a global, multiple instantiations of the same kernel (with different concrete params) will race (on that global TypeVar). TypeVars in the closure don't suffer from this because we copy the body_builder and its closure when instantiating the kernel. In 3.12 (when using the brackets def fun[M, N, ...] syntax) the TypeVars are always in the closure¹. I tried to figure out somehow to move globals into closures but failed and decided it's not worth the effort anyway.

Note, the added test (test_generics) is gated behind 3.13 because generics default value assignment is a 3.13 feature and the 3.12 parser will choke on it, but everything else works in 3.12.

Footnotes

1. That's actually a CPython implementation detail - there's a compiler transform which rewrites the function into a closed over function with those TypeVars in its closure. ↩