Header files of the Include/cpython/ subdirectory now get a
"cpython_" prefix in their filename to avoid confusion between
Include/<header>.h and Include/cpython/<header.h> (previously both
files had the same name), especially when a cpython header file
includes another cpython header file. Headers of the Include/cpython/
subdirectory cannot be included directly, so this change has no
impact on the C API.
cpython_pystate.h now includes "cpython_initconfig.h" rather than
"cpython/initconfig.h" to fix building Python on macOS Framework.