Co-authored-by: Tao Xu <tao@openai.com>

* Add GCC9 and prefer newer over older versions

* Bump version

* a small fix

* make it or

Move from `lockfile` library to `fasteners`

* PID implementation inside mujoco-py

* addressing the comments from Jerry and Arthur

* add unit tests for new PID control

* add one more unit test for backward compatibility

* addressing comments

Also point to mujoco-bin for licenses

* Reraising mujoco error exceptions

Mujoco 2.0.2.3 

* Initialize perturbations properly.

* Expose "set constants" mujoco operation.

* Updated README documentation.

* Removed the 'alpha' pre-release version info.

* Fixing pip install command that seems to help with docker build in CI.

* Updating README to the next major version.

* improvements to sim.ray()

* typo

* group filter is bytes