* Split long.c tests of _testcapi into two parts: limited C API tests
in _testlimitedcapi and non-limited C API tests in _testcapi.
* Move testcapi_long.h from Modules/_testcapi/ to
Modules/_testlimitedcapi/.
* Add MODULE__TESTLIMITEDCAPI_DEPS to Makefile.pre.in.
Make sure that the internal C API is not tested by mistake by
_testcapi.
Undefine Py_BUILD_CORE_BUILTIN and Py_BUILD_CORE_MODULE macros in
Modules/_testcapi/parts.h: move code from _testcapimodule.c.
heaptype_relative.c and vectorcall_limited.c are using the limited C
API which is incompatible with the internal C API.
Move test_long_numbits() from _testcapi to _testinternalcapi since it
uses the internal C API "pycore_long.h".
Fix Modules/_testcapi/pyatomic.c: don't include Python.h directly,
just include _testcapi/parts.h.
Ajust "make check-c-globals" for these changes.
Move the private _PyLong_Sign() and _PyLong_NumBits() functions
to the internal C API (pycore_long.h).
Modules/_testcapi/long.c now uses the internal C API.