Add pyproject.toml for build dependencies

[bchess]

setup.py's build does an actual compilation of mujoco-py, which has dependencies. These are thus build dependencies, not just install dependencies. Right now, oftentimes when doing pip install mujoco-py, this build fails and the compilation doesn't occur until actual first runtime import.

PEP518 describes the pyproject.toml file used to declare build dependencies. This adds that file, fixing the behavior to compile mujoco-py during pip install.

[MillionIntegrals]

Looks good, one question: do setuptools/pip automatically process pyproject.toml or is it more of an information for users?

[MillionIntegrals]

👍

[bchess]

Looks good, one question: do setuptools/pip automatically process pyproject.toml or is it more of an information for users?

It automatically uses it.