Fix installing from pip by adding setup requirements.

[PavelCz]

As can be seen here, procgen requires gym3 during build. Now you might think that's no problem since gym3 is in the dependencies, but these dependencies are actually installed after the build. When moving the gym3 requirement into the setup_requires dependencies, this change allows people to directly install procgen using e.g. pip like so: pip install "procgen @ git+https://github.com/JacobPfau/procgenAISC.git", without needing to manually install gym3 before or something like that.

[PavelCz]

@langosco This could be potentially helpful for people using procgen in their requirements.