Issue #22463: Backport compiler warning fixes and workarounds

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.