matplotlib.docstring is deprecated

Warnings are being emitted from matplotlib that will lead to an incompatibility very soon [ref]:

gwpy/__init__.py:34: in <module>
    from . import (
gwpy/plot/__init__.py:29: in <module>
    from . import (
gwpy/plot/gps.py:28: in <module>
    from matplotlib import (ticker, docstring)
../../../miniconda3/envs/test/lib/python3.8/site-packages/matplotlib/docstring.py:3: in <module>
    _api.warn_deprecated(
../../../miniconda3/envs/test/lib/python3.8/site-packages/matplotlib/_api/deprecation.py:96: in warn_deprecated
    warn_external(warning, category=MatplotlibDeprecationWarning)
../../../miniconda3/envs/test/lib/python3.8/site-packages/matplotlib/_api/__init__.py:363: in warn_external
    warnings.warn(message, category, stacklevel)
E   matplotlib._api.deprecation.MatplotlibDeprecationWarning: The matplotlib.docstring module was deprecated in Matplotlib 3.6 and will be removed two minor releases later.