Switch from pathlib to pathlib2

The pathlib backport module is no longer maintained. The development
has moved to the pathlib2 module instead.

Quoting from the pathlib's README:
"Attention: this backport module isn't maintained anymore. If you want
to report issues or contribute patches, please consider the pathlib2
project instead."

tests/conftest.py

@@ -1,4 +1,7 @@
-from pathlib import Path
+try:
+    from pathlib import Path
+except ImportError:
+    from pathlib2 import Path
 import pytest
 from thefuck import shells
 from thefuck import conf, const