TST: matplotlib 3.4.1 broke RTD with MatplotlibDeprecationWarning

You will see this error on CircleCI for mpldev and in the dev job of Actions:

INTERNALERROR> ValueError: Key backend: '' is not a valid value for backend; supported values are
['GTK3Agg', 'GTK3Cairo', 'MacOSX', 'nbAgg', 'Qt4Agg', 'Qt4Cairo', 'Qt5Agg', 'Qt5Cairo',
'TkAgg', 'TkCairo', 'WebAgg', 'WX', 'WXAgg', 'WXCairo', 'agg', 'cairo',
'pdf', 'pgf', 'ps', 'svg', 'template']

You will see these warnings on RTD:

MatplotlibDeprecationWarning:
The validCap attribute was deprecated in Matplotlib 3.4 and will be removed two minor releases later.
  return getattr(obj, attr, *defargs)
MatplotlibDeprecationWarning:
The validJoin attribute was deprecated in Matplotlib 3.4 and will be removed two minor releases later.
  return getattr(obj, attr, *defargs)

~pandas saw something similar for the warning and their fix is pandas-dev/pandas#32444~

TODO

• Revert #11466 and #11468 when matplotlib/matplotlib#19839 is resolved and that fix is released.
• Wait for fix for matplotlib/matplotlib#19848 (the fix in their dev should be sufficient). Follow up at #11471

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[nstarman]

@pllim. I removed that part of the patch because I found it didn’t have an effect. The matplotlib docs use `.CapStyle`, which as a relative path can’t be overridden or ignored by numpydoc xref. Thanks, I’ll open a PR