Support the free-threaded build with python backend #975
Description
Currently, it fails: #974 (comment)
I suspect, that this mostly coming from using global caches, e.g.:
mpmath/mpmath/libmp/libelefun.py
Lines 46 to 50 in 5d66b10
See https://py-free-threading.github.io/porting/#dealing-with-mutable-global-state
We also use lru_cache(), which is not thread-safe, until recently.