CI: Include Python.h first in _tkagg.cpp

Needs to be included before system headers to set visibility macros for pybind11

Author: DWesl

src/_tkagg.cpp

@@ -9,6 +9,7 @@
 // rewritten, we have removed the PIL licensing information.  If you want PIL,
 // you can get it at https://python-pillow.org/
 
+#include <Python.h>
 #include <memory>
 #include <new>
 #include <stdexcept>