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.
BUG: fix memoryleaks in datetime dtype parsing

closes gh-4973
closes gh-4974
Merge pull request #4975 from juliantaylor/datetime-memleak

BUG: fix memoryleaks in datetime dtype parsing
Correct the fill_diagonal examples
Merge pull request #4981 from hpaulj/fill_diagonal

Correct the fill_diagonal examples