Support openai gym>=0.25.0

Since the release of gym==0.25.0, gym_guppy tests are failing with:

ImportError while loading conftest '/builds/bioroboticslab/robofish/gym-guppy/tests/conftest.py'.
tests/conftest.py:2: in <module>
    from robofish.gym_guppy.envs import GuppyEnv
/usr/lib/python3.8/site-packages/robofish/gym_guppy/__init__.py:4: in <module>
    from .envs import GuppyEnv, GoalGuppyEnv, VariableStepGuppyEnv, VariableStepGoalGuppyEnv
/usr/lib/python3.8/site-packages/robofish/gym_guppy/envs/__init__.py:21: in <module>
    from ._guppy_env import GuppyEnv
/usr/lib/python3.8/site-packages/robofish/gym_guppy/envs/_guppy_env.py:25: in <module>
    class GuppyEnv(gym.Env, metaclass=abc.ABCMeta):
E   TypeError: metaclass conflict: the metaclass of a derived class must be a (non-strict) subclass of the metaclasses of all its bases

gym==0.24.1 is the last version that works.