apply pragma system_header to python headers

Analogous to 388e80132c (which was for Perl) but for Python, I propose 
adding #pragma GCC system_header to plpython.h.  Without it, you get 
tons of warnings about -Wdeclaration-after-statement, starting with 
Python 3.12.  (In the past, I have regularly sent feedback to Python to 
fix their header files, but this is getting old, and we have an easier 
solution now.)