Make python-forge an optional/dev dependency only

[JWCook]

Forge is currently used to automate modifying function signatures with additional keyword arguments (including type annotations and docstrings). For example, adding all Redis-specific connection parameters to the docs for RedisBackend. This affects both Sphinx docs (example) and IDE autocompletion.

This seemed like a good idea at the time, but I don't think it's worth an extra (required) dependency, especially since it doesn't appear to be maintained anymore. I'm thinking of removing most of the usage of forge, and only keeping the part used for adding the signature of aiohttp.ClientSession to CachedSession for Sphinx doc builds.