TST: suppress matplotlib warnings (or determine if they represent a bug)

[jbrockmendel]

There are a bunch of these in the test logs:

pandas/tests/plotting/test_hist_method.py::TestDataFramePlots::test_hist_column_order_unchanged[None-expected0]
pandas/tests/plotting/test_hist_method.py::TestDataFramePlots::test_hist_column_order_unchanged[column1-expected1]
pandas/tests/plotting/test_hist_method.py::TestDataFramePlots::test_hist_with_legend[None-None]
   pandas/tests/plotting/common.py:556: UserWarning: To output multiple subplots, the figure containing the passed axes is being cleared
    ret = f(**kwargs)

Do these represent problems we need to do something about? cc @tacaswell

If not, we should suppress them. My attempts to do so with pytestmark = pytest.mark.filterwarnings have failed, could use another pair of eyeballs.

[tacaswell]

The warnings are from the pandas side of things:

pandas/pandas/plotting/_matplotlib/tools.py

Lines 226 to 231 in 58dcafa

	warnings.warn(
	"To output multiple subplots, the figure containing "
	"the passed axes is being cleared",
	UserWarning,
	stacklevel=4,
	)

[fangchenli]

xref #18306

[ivanovmg]

I fixed similar warnings in #36982
I can look at the remaining ones of this kind.

[jbrockmendel]

@ivanovmg is this closed by #37274?

[ivanovmg]

@jbrockmendel I think the PR mentioned helps resolve the problem.
However, let me check whether there are still some tests issuing the same warning.
If so, then I will apply the same changes as was already done in the PR.

[ivanovmg]

I checked on master.
No more warnings of this kind:

UserWarning: To output multiple subplots, the figure containing the passed axes is being cleared

So, probably the issue can be closed.

[jbrockmendel]

thanks