gym and pygame dependencies missing from setup.py requirements

After
```
git clone git@github.com:vitchyr/multiworld.git
cd multiworld
pip install -e .
python  # version 3.8.0
>>> import multiworld
```
I get
```
ModuleNotFoundError: No module named 'gym'
```

After i `pip install gym`, I get
```
ModuleNotFoundError: No module named 'pygame'
```