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.)