* Add missing includes.
* Remove unused includes.
* Update old include/symbol names to newer names.
* Mention at least one included symbol.
* Sort includes.
* Update Tools/cases_generator/generate_cases.py used to generated
pycore_opcode_metadata.h.
* Update Parser/asdl_c.py used to generate pycore_ast.h.
* Cleanup also includes in _testcapimodule.c and _testinternalcapi.c.
Remove private pylifecycle.h functions: move them to the internal C
API ( pycore_atexit.h, pycore_pylifecycle.h and pycore_signal.h). No
longer export most of these functions.
Move _testcapi.test_atexit() to _testinternalcapi.
Fix signal.NSIG value on FreeBSD to accept signal numbers greater
than 32, like signal.SIGRTMIN and signal.SIGRTMAX.
* Add Py_NSIG constant.
* Add pycore_signal.h internal header file.
* _Py_Sigset_Converter() now includes the range of valid signals in
the error message.