Start moving everything to the new API

This isn't a valid marker apparently.

From jenkins output:
```
==================================== ERRORS ====================================
________ ERROR collecting tests/hazmat/primitives/test_serialization.py ________
tests/hazmat/primitives/test_serialization.py:33: in <module>
    @pytest.mark.pem_serialization
.tox/py27/lib/python2.7/site-packages/_pytest/mark.py:174: in __getattr__
    self._check(name)
.tox/py27/lib/python2.7/site-packages/_pytest/mark.py:189: in _check
    raise AttributeError("%r not a registered marker" % (name,))
E   AttributeError: 'pem_serialization' not a registered marker
=========================== short test summary info ============================
```