Revert "Python: reduce amount of warnings when building with buildPython*"

Turns out that many packages' testsuites rely on the default warnings setup.

This reverts commit c1e003ff2f.

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

@@ -101,10 +101,6 @@ 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;

pkgs/development/python-modules/pytest/default.nix

@@ -24,8 +24,7 @@ buildPythonPackage rec {
 
   checkPhase = ''
     runHook preCheck
-    # Re-enable warnings because the test suite relies on it.
-    PYTHONWARNINGS= $out/bin/py.test -x testing/
+    $out/bin/py.test -x testing/
     runHook postCheck
   '';