pythonPackages.cryptography: ignore pytest warnings

The test suite was generating a lot of warnings, causing the hydra build
to fail. Unfortunately, PYTHONWARNINGS env var is completely ignored.

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

@@ -51,6 +51,10 @@ buildPythonPackage rec {
     hypothesis
   ];
 
+  checkPhase = ''
+    py.test --disable-pytest-warnings tests
+  '';
+
   # The test assumes that if we're on Sierra or higher, that we use `getentropy`, but for binary
   # compatibility with pre-Sierra for binary caches, we hide that symbol so the library doesn't
   # use it. This boils down to them checking compatibility with `getentropy` in two different places,