FIX: fix cleanup warnings for errorbar timeseries
- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] passes `black pandas`
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

Currently in ``pandas/tests/plotting/tests_frame.py`` there are several warnings emitted.

```
pandas/tests/plotting/test_frame.py::TestDataFramePlots::test_mpl2_color_cycle_str
  /workspaces/pandas/pandas/plotting/_matplotlib/style.py:64: 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_errorbar_plot
pandas/tests/plotting/test_frame.py::TestDataFramePlots::test_errorbar_plot
pandas/tests/plotting/test_frame.py::TestDataFramePlots::test_errorbar_plot
pandas/tests/plotting/test_frame.py::TestDataFramePlots::test_errorbar_timeseries
pandas/tests/plotting/test_frame.py::TestDataFramePlots::test_errorbar_timeseries
pandas/tests/plotting/test_frame.py::TestDataFramePlots::test_errorbar_timeseries
  /workspaces/pandas/pandas/plotting/_matplotlib/__init__.py:61: UserWarning: To output multiple subplots, the figure containing the passed axes is being cleared
    plot_obj.generate()

-- Docs: https://docs.pytest.org/en/stable/warnings.html
```

This PR handles warnings in ``test_errorbar_timeseries``.
If this approach is considered reasonable by the reviewers, then I will do the same for ``test_errorbar_plot`` as well as other similar warnings among the plotting-related tests.