test that python modules nominally compile without warnings

Because we poke around with the dependency, so we might introduce some,
and have at least once.

test cases/python/2 extmodule/meson.build

@@ -1,5 +1,5 @@
 project('Python extension module', 'c',
-  default_options : ['buildtype=release'])
+  default_options : ['buildtype=release', 'werror=true'])
 # Because Windows Python ships only with optimized libs,
 # we must build this project the same way.