Release notes

• Added rendering for CliffWalking environment @younik
• PixelObservationWrapper only supports the new render API due to difficulty in supporting both old and new APIs. A warning is raised if the user is using the old API @vmoens

Bug fix

• Revert an incorrect edition on wrapper.FrameStack @ZhiqingXiao
• Fix reset bounds for mountain car @psc-g
• Removed skipped tests causing bugs not to be caught @pseudo-rnd-thoughts
• Added backward compatibility for environments without metadata @pseudo-rnd-thoughts
• Fixed BipedalWalker rendering for RGB arrays @1b15
• Fixed bug in PixelObsWrapper for using the new rendering @younik

Typos

• Rephrase observations' definition in Lunar Lander Environment @EvanMath
• Top-docstring in gym/spaces/dict.py @Ice1187
• Several typos in humanoidstandup_v4.py, mujoco_env.py, and vector_list_info.py @timgates42
• Typos in passive environment checker @pseudo-rnd-thoughts
• Typos in Swimmer rotations @lin826