Whithout the prePatch block, I get the following error on Python 3:

======================================================================
ERROR: test (unittest.loader._FailedTest)
----------------------------------------------------------------------
ImportError: Failed to import test module: test
Traceback (most recent call last):
  File "/nix/store/4wqa1whb42khic67rv9jgip30nxmaii8-python3-3.6.4/lib/python3.6/unittest/loader.py", line 153, in loadTestsFromName
    module = __import__(module_name)
  File "/tmp/nix-build-python3.6-fritzconnection-0.6.5.drv-0/fritzconnection-0.6.5/fritzconnection/test.py", line 16, in <module>
    from fritzconnection import (
ImportError: cannot import name 'FritzXmlParser'

But as it is, building and testing works fine on both, Python 2 and 3.

Interesting. There is a package called fritzconnection, and it has a module inside it called fritzconnection. The module contains the FritzXmlParser class. This class is not exposed by the package, so the only way to get it is to import from the module directly.

Packages and modules in the current folder take precedence over PYTHONPATH, so in this case it will load the fritzconnection module first. However, in Python 3 such an import is not allowed, and so it will load the package instead and fail. Using .fritzconnection works because this is a relative import.

To conclude, upstream likely does not test against Python 3.

Thanks for the explanation, @FRidh. I'll open an upstream issue asap.

See https://bitbucket.org/kbr/fritzconnection/pull-requests/13.
Since the project doesn't seem very active, I don't think this will get merged any time soon.