pytest migration: make pytest's parametrize functionality available

To provide parameterized tests, a custom implementation is currently
provided at kallithea/tests/parameterized.py because nose does not provide
that out-of-the-box.
pytest, on the other hand, does have a built-in 'parametrize' (note:
different spelling) functionality. Therefore, once all tests have been
converted to pytest, we can get rid of the custom 'parameterized'
implementation. Also, the existing 'parameterized' implementation does not
seem to work under pytest-style tests.

This commit makes pytest's 'parametrize' decorator available alongside the
custom 'parameterized' decorator. The names are confusing but it is the
intention to kill the original 'parameterized' soon.