bpo-47152: Automatically regenerate sre_constants.h (GH-91439)

* Move the code for generating Modules/_sre/sre_constants.h from
  Lib/re/_constants.py into a separate script
  Tools/scripts/generate_sre_constants.py.
* Add target `regen-sre` in the makefile.
* Make target `regen-all` depending on `regen-sre`.