Python: reduce amount of warnings when building with buildPython*

By default all warnings were printed. This occasionally resulted in
a lot of warnings leading to builds being killed.

This commit reduces the amount of warnings printed.

pkgs/development/interpreters/python/mk-python-derivation.nix

@@ -101,6 +101,10 @@ toPythonModule (python.stdenv.mkDerivation (builtins.removeAttrs attrs [
     ${python.interpreter} ${./catch_conflicts}/catch_conflicts.py
   '' + attrs.postFixup or '''';
 
+  # Print fewer warnings so we have less noise in our logs
+  # The amount of warnings also caused builds to be terminated.
+  PYTHONWARNINGS="once";
+
   meta = {
     # default to python's platforms
     platforms = python.meta.platforms;