0.3.7 - add "get_communities" method

Outlined in PR #21, this change adds the get_communities method to the LemmyHttp object. For example:

from plemmy import LemmyHttp

srv = LemmyHttp("<lemmy server>")
api_response = srv.get_communities(type_="All", sort="Hot", page=0, limit=50)

Thanks to @gwbischof for the contribution!