Issue #22463: Backport compiler warning fixes and workarounds

* Set but unused variable in Parser/pgen.c in non-debug builds. Patch by
  Christian Heimes.
* Unused static function in Modules/readline.c. Patch by Georg Brandl.
* main_window unused in Modules/tkappinit.c. Patch by Gregory P. Smith.
* Dead assignment in Modules/_ctypes/cfield.c. Extracted from patch by Brett
  Cannon.
* Expression result unused in PyObject_INIT macro expansions. Based on
  patches by Christian Heimes.
* Load expat_config.h and therefore pyconfig.h before C stdlib headers are
  loaded. This silences pre-processor warnings including '_POSIX_C_SOURCE
  redefined'. Extracted from patch by Christian Heimes.
diff --git a/Objects/unicodeobject.c b/Objects/unicodeobject.c
index 0b81a3c..ca6628e 100644
--- a/Objects/unicodeobject.c
+++ b/Objects/unicodeobject.c
@@ -347,7 +347,7 @@
             size_t new_size = sizeof(Py_UNICODE) * ((size_t)length + 1);
             unicode->str = (Py_UNICODE*) PyObject_MALLOC(new_size);
         }
-        PyObject_INIT(unicode, &PyUnicode_Type);
+        (void)PyObject_INIT(unicode, &PyUnicode_Type);
     }
     else {
         size_t new_size;