Add a private list of all stdlib modules: _Py_module_names.
* Add Tools/scripts/generate_module_names.py script.
* Makefile: Add "make regen-module-names" command.
* setup.py: Add --list-module-names option.
* GitHub Action and Travis CI also runs "make regen-module-names",
not ony "make regen-all", to ensure that the module names remains
up to date.
