The headers define a number of identifiers that don't start with Py (or _Py), potentially clashing with other code.

• slot function typedefs like destructor or setter
• calling convention names like METH_VARARGS
• non-typedef'd structs, like struct _object
• configure cruft like HAVE_STRFTIME or SIZEOF_SIZE_T
• and more

Relatedly, header names should probably also be namespaced (or hidden in a subdirectory, expecially if users aren't supposed to include them directly)