It's system-dependent. stubdefaulter told me `multiprocessing.heap.Heap: parameter size: stub default 4096 != runtime default 16384`, which presumably means it's 4096 on @AlexWaygood's system and 16384 on mine. I looked in the code and the default is set to `mmap.PAGESIZE`, which in turn is set from some system call at import time.
Continuing work towards #8988.
The first five commits were created using stubdefaulter on various Python versions; the following commits were all created manually by me to fix various problems. The main things this adds that weren't present in #9501 are:
- Defaults in Windows-only modules and Windows-only branches (because I'm running a Windows machine)
- Defaults in non-py311 branches
- Defaults for float parameters
- Defaults for overloads
