use mujoco-py-cython3 to retain compatibility

[Kallinteris-Andreas]

Description

https://github.com/Kallinteris-Andreas/mujoco-py is a soft fork mujoco-py that simply changes the build flags

if this gets merged, we can delay #993 the removal of mujoco-py environments for 1-2 years

this will also be merged in gymnasium-robotics

Alternative: simply remove mujoco-py #993

Type of change

Please delete options that are not relevant.

• Documentation only change (no code changed)
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update