MAINT: Get rid of some harmless gcc warnings about macro redefines.

The Python.h header likes to be included first, otherwise warnings about
redefining "_POSIX_C_SOURCE" and "_XOPEN_SOURCE" are generated. This is
really a problem with the Python.h header, but can be worked around by
changing the order of inclusion.
Merge pull request #4971 from charris/fix-redefined-macro-warnings

MAINT: Get rid of some harmless gcc warnings about macro redefines.