Use pathlib for git-graph python module

[neodark]

To run this package as a Python module will crash as explained in #3

Since Python 3 it's a good practice to use Pathlib when needed.

This closes #3

[neodark]

All tests are now passing but it's failing with Travis CI due to flake8. Will try to correct that later.

[neodark]

Fixed the issue with Travis CI due to flake8 used with incorrect python version.

Now all is green and working again. I will let you review the fixes and merge them and bump a new pypi package.

Have a nice day! :)