bpo-38075: Port _randommodule.c to PEP-384 (GH-15798)

- Migrate `Random_Type` to `PyType_FromSpec`
- To simulate an old use of `PyLong_Type.tp_as_number->nb_absolute`, I added
  code to the module init function to stash `int.__abs__` for later
  use. Ideally we'd use `PyType_GetSlot()` instead, but it doesn't currently
  work for static types in CPython, and implementing it just for this case
  doesn't seem worth it.
- Do exact check for long and dispatch to PyNumber_Absolute, use vector call when not exact.