Environs

- Add `subcast_key` argument to `env.dict` ([151](https://github.com/sloria/environs/issues/151)).
Thanks [AugPro](https://github.com/AugPro) for the suggestion and PR.

- Fix error when parsing empty list with subcast
[137](https://github.com/sloria/environs/issues/137).
Thanks [sabdouni] for the catch and patch.

- `log_level` accepts lower-cased log level names and rejects invalid
names ([138](https://github.com/sloria/environs/pull/138)).
Thanks [gnarvaja](https://github.com/gnarvaja) for the PR.

- Add `dj_cache_url` for caching Django cache URLs (requires installing with `[django]`)
([126](https://github.com/sloria/environs/issues/126)).
Thanks [epicserve](https://github.com/epicserve) for the suggestion and PR.

- Improve typings and run mypy with dependencies type annotations ([115](https://github.com/sloria/environs/pull/115)).
- Distribute types per PEP 561 ([116](https://github.com/sloria/environs/pull/116)).

Thanks [hukkinj1](https://github.com/hukkinj1) for the PRs.

- _Backwards-incompatible_: Remove `stream` argument from `read_env`,
since it had no effect ([114](https://github.com/sloria/environs/pull/114)).
- _Backwards-incompatible_: `Env.read_env` consistently returns `None`
([111](https://github.com/sloria/environs/pull/111)).
- Remove unnecessary `__str__` definition ([112](https://github.com/sloria/environs/pull/112)).

Thanks [hukkinj1](https://github.com/hukkinj1) for the PRs.