Implement rudimentary compatibility with jedi 18.0

[seppeljordan]

PR Summary

This PR implements basic compatibility with jedi 18.0. The issue was that the default json serializer used by python json.dumps could not handle pathlib.Path objects. This PR addresses the issue by extending the default JSONEncoder class. pathlib.Path objects are converted to str before encoding them to json.

PR checklist

Please make sure that the following things have been addressed (and check the relevant checkboxes):

• Commits respect our guidelines
• Tests are passing properly (see here on how to run Elpy's tests)

Test are not really passing BUT the tests were not passing before this change anyway. At least some more tests are passing now.

[galaunay]

Duplicate of #1902.

#1895 will probably overwrite those modifications, but in the meantime, it will be nice to have jedi 0.18 working.
Thanks