Make dynamic patcher work with pytest

[mrbean-bremen]

This is a followup for #248.
It would be nice to have the dynamic patcher be activated by default (instead of being an experimental feature), but currently it does not work with pytest (also has to be checked with nose). The problem is that both pytest and the dynamic patcher manipulate sys.modules, and these changes do not work together nicely.
The other probem was that it didn't work for the Python 3.2 based PyPy3 2.4 - I consider this a non-issue, as we no longer support Python 3.2.