Add pathlib2 to install_requirements for python_version < 3.4 (#2205)

The dynamic addition of the dependency which was used previously wasn't
included in the published egg on pypi.

setup.py

@@ -65,11 +65,9 @@
                         'six>=1.12.0',
                         'rpyc',
                         'colored_traceback',
+                        'pathlib2; python_version < "3.4"',
 ]
 
-if platform.python_version_tuple()[0] == '2':
-    install_requires += ['pathlib2']
-
 # Check that the user has installed the Python development headers
 PythonH = os.path.join(get_python_inc(), 'Python.h')
 if not os.path.exists(PythonH):