… turtle (#126350)

Co-authored-by: Marie Roald <[email protected]>
Co-authored-by: Yngve Mardal Moe <[email protected]>
Co-authored-by: Hugo van Kemenade <[email protected]>
Co-authored-by: Daniel Hollas <[email protected]>
Co-authored-by: Bénédikt Tran <[email protected]>
Co-authored-by: Erlend E. Aasland <[email protected]>

This reduces the import time of the `csv` module by up to five times,
by importing `re` on demand.

In particular, the `re` module is no more implicitly exposed as `csv.re`.

Rename PYTHREAD_NAME_MAXLEN to _PYTHREAD_NAME_MAXLEN.

There is a race condition between PyMem_SetAllocator() and
PyMem_RawMalloc()/PyMem_RawFree(). While PyMem_SetAllocator() write
is protected by a lock, PyMem_RawMalloc()/PyMem_RawFree() reads are
not protected by a lock. PyMem_RawMalloc()/PyMem_RawFree() can be
called with an old context and the new function pointer.

On a release build, it's not an issue since the context is not used.
On a debug build, the debug hooks use the context and so can crash.