chore: Add support for defining PYTHONQT_USE_PYSTRING_SHIM

Author: jcfr

src/PythonQtPythonInclude.h

@@ -121,9 +121,12 @@
 #error "PythonQt requires Python >= 3.x"
 #endif
 
+// Optional compatibility shim for legacy wrappers generated by older PythonQt.
+// Enable by defining PYTHONQT_USE_PYSTRING_SHIM (deprecated).
+#if defined(PYTHONQT_USE_PYSTRING_SHIM)
 #define PY3K
-// Helper defines to facilitate porting
 #define PyString_FromString PyUnicode_FromString
+#endif
 
 // Avoid clashes with libstdc++ <locale> by undefining ctype macros
 // that CPython may introduce on macOS when the UTF-8 ctype quirk is enabled.