TST: avoid matplotlib warnings in tests
In master we get the following warnings in this file:

```
$ pytest pandas/tests/plotting/test_frame.py --skip-slow
[...]

pandas/tests/plotting/test_frame.py::TestDataFramePlots::test_mpl2_color_cycle_str
  pandas/pandas/plotting/_style.py:60: MatplotlibDeprecationWarning: Support for uppercase single-letter colors is deprecated since Matplotlib 3.1 and will be removed in 3.3; please use lowercase instead.
    [conv.to_rgba(c) for c in colors]

pandas/tests/plotting/test_frame.py::TestDataFramePlots::test_subplots_timeseries_y_axis
  pandas/pandas/plotting/_core.py:384: FutureWarning: Converting timezone-aware DatetimeArray to timezone-naive ndarray with 'datetime64[ns]' dtype. In the future, this will return an ndarray with 'object' dtype where each element is a 'pandas.Timestamp' with the correct 'tz'.
  	To accept the future behavior, pass 'dtype=object'.
  	To keep the old behavior, pass 'dtype="datetime64[ns]"'.
    numeric_data[col] = np.asarray(numeric_data[col])

pandas/tests/plotting/test_frame.py::TestDataFramePlots::test_line_area_stacked
pandas/tests/plotting/test_frame.py::TestDataFramePlots::test_line_area_stacked
pandas/tests/plotting/test_frame.py::TestDataFramePlots::test_line_area_stacked
pandas/tests/plotting/test_frame.py::TestDataFramePlots::test_line_area_stacked
pandas/tests/plotting/test_frame.py::TestDataFramePlots::test_line_area_stacked
pandas/tests/plotting/test_frame.py::TestDataFramePlots::test_line_area_stacked
pandas/tests/plotting/test_frame.py::TestDataFramePlots::test_line_area_stacked
pandas/tests/plotting/test_frame.py::TestDataFramePlots::test_line_area_stacked
pandas/tests/plotting/test_frame.py::TestDataFramePlots::test_line_area_stacked
pandas/tests/plotting/test_frame.py::TestDataFramePlots::test_line_area_stacked
pandas/tests/plotting/test_frame.py::TestDataFramePlots::test_line_area_stacked
pandas/tests/plotting/test_frame.py::TestDataFramePlots::test_line_area_stacked
pandas/tests/plotting/test_frame.py::TestDataFramePlots::test_line_area_stacked
pandas/tests/plotting/test_frame.py::TestDataFramePlots::test_line_area_stacked
pandas/tests/plotting/test_frame.py::TestDataFramePlots::test_line_area_stacked
pandas/tests/plotting/test_frame.py::TestDataFramePlots::test_line_area_stacked
  pandas/pandas/plotting/_core.py:1014: UserWarning: Attempted to set non-positive left xlim on a log-scaled axis.
  Invalid limit will be ignored.
    ax.set_xlim(left, right)
```

This PR addresses first and third of these.  The middle one is the topic of #26676.