BLD,Cygwin: Included Python.h first in src/_c_internal_utils.cpp

Python.h needs to be included before any system includes.  This is easiest if it is the first include.  Interestingly, pybind11/pybind11.h does not include Python, possibly depending on downstream projects including Python.h beforehand.

Author: DWesl

src/_c_internal_utils.cpp

@@ -1,3 +1,6 @@
+#include <Python.h>
+/* Python.h must be included before any system headers,
+    to ensure visibility macros are properly set. */
 #include <stdexcept>
 
 #ifdef _WIN32