Don't require pathlib for Python >= 3.4

[cbrnr]

Python >= 3.4 ships pathlib with its standard library. Furthermore, the pathlib package on PyPI is not maintained anymore. Instead, they recommend to use the pathlib2 package. Therefore, the entry in setup.py should be modified as follows (https://stackoverflow.com/a/32643122/1112283):

'pathlib2;python_version<"3.4"'

The package import should then also be changed to something along the following lines:

try:
    import pathlib
except ImportError:
    import pathlib2 as pathlib