Add `isolate_method` context manager to utils.py

* Add `next_mro_class` and `import_path` helper functions
Refactor `test_isolate_method`

* Instantiate `isolate_method` before using it as a context manager
Add python 2 compatibility
Merge pull request #46 from incuna/isolate-method

Add `isolate_method` context manager to utils.py