Mock plt.subplots which has multiple outputs

[hunse]

plt.subplots could not be mocked out when used in the common form fig, axes = plt.subplots(...), since our mock objects just return one Mock output which could not be split. I've started a registry of functions that return multiple outputs (with plt.subplots currently being the only one), so that we can return the proper number of mock objects.

[github-actions]

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pytest_plt/version.py

100% of new lines are covered (100% of the complete file).

pytest_plt/plugin.py

100% of new lines are covered (100% of the complete file).