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The pathlib backport module is no longer maintained.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."

setup.py

@@ -29,7 +29,7 @@ elif (3, 0) < version < (3, 3):
 VERSION = '3.9'
 
 install_requires = ['psutil', 'colorama', 'six', 'decorator']
-extras_require = {':python_version<"3.4"': ['pathlib'],
+extras_require = {':python_version<"3.4"': ['pathlib2'],
                   ":sys_platform=='win32'": ['win_unicode_console']}
 
 setup(name='thefuck',