BUG: Strange behavior at testing test_ts_plot_with_tz
I came across a rather strange and annoying behavior when running the plotting tests that leaves me completely clueless.   

`TestTSPlot.test_ts_plot_with_tz['UTC']` fails when running the whole test suite but passes when run in isolation. In particular it fails when run **after all** `TestRegistration` tests and passes when run **on its own or after any** of the six test functions in `TestRegistration`:

### FAILURE:
```
(pandas-dev) C:\Daten\Python\pandas>pytest pandas\tests\plotting\ -k "TestRegistration | test_ts_plot_with_tz"
====================================================================== test session starts ======================================================================
platform win32 -- Python 3.8.2, pytest-5.4.3, py-1.9.0, pluggy-0.13.1
rootdir: C:\Daten\Python\pandas, inifile: setup.cfg
plugins: hypothesis-5.19.0, asyncio-0.14.0, cov-2.10.0, forked-1.2.0, xdist-1.32.0
collected 479 items / 460 deselected / 19 selected

pandas\tests\plotting\test_converter.py ......                                                                                                             [ 31%]
pandas\tests\plotting\test_datetimelike.py F............                                                                                                   [100%]

=========================================================================== FAILURES ============================================================================
____________________________________________________________ TestTSPlot.test_ts_plot_with_tz['UTC'] _____________________________________________________________

self = <pandas.tests.plotting.test_datetimelike.TestTSPlot object at 0x000002965C714E20>, tz_aware_fixture = 'UTC'

    @pytest.mark.slow
    def test_ts_plot_with_tz(self, tz_aware_fixture):
        # GH2877, GH17173, GH31205, GH31580
        tz = tz_aware_fixture
        index = date_range("1/1/2011", periods=2, freq="H", tz=tz)
        ts = Series([188.5, 328.25], index=index)
        with tm.assert_produces_warning(None):
            _check_plot_works(ts.plot)
            ax = ts.plot()
            xdata = list(ax.get_lines())[0].get_xdata()
            # Check first and last points' labels are correct
>           assert (xdata[0].hour, xdata[0].minute) == (0, 0)
E           AttributeError: 'numpy.datetime64' object has no attribute 'hour'

pandas\tests\plotting\test_datetimelike.py:57: AttributeError
==================================================================== short test summary info ====================================================================
FAILED pandas/tests/plotting/test_datetimelike.py::TestTSPlot::test_ts_plot_with_tz['UTC'] - AttributeError: 'numpy.datetime64' object has no attribute 'hour'
========================================================= 1 failed, 18 passed, 460 deselected in 2.98s ==========================================================
```
### SUCCESS:
```
(pandas-dev) C:\Daten\Python\pandas>pytest pandas\tests\plotting\ -k test_ts_plot_with_tz
====================================================================== test session starts ======================================================================
platform win32 -- Python 3.8.2, pytest-5.4.3, py-1.9.0, pluggy-0.13.1
rootdir: C:\Daten\Python\pandas, inifile: setup.cfg
plugins: hypothesis-5.19.0, asyncio-0.14.0, cov-2.10.0, forked-1.2.0, xdist-1.32.0
collected 479 items / 466 deselected / 13 selected

pandas\tests\plotting\test_datetimelike.py .............                                                                                                   [100%]

============================================================== 13 passed, 466 deselected in 2.43s ===============================================================
```
As I said it passes when run after any of the individual functions in  `TestRegistration`, i.e. with `-k "test_register_by_default | test_ts_plot_with_tz"`, `"test_registering_no_warning | test_ts_plot_with_tz"` etc.    

Tests are run in an environment set up as described [here](https://pandas.pydata.org/pandas-docs/stable/development/contributing.html#creating-a-python-environment-pip). Pytest and plugin versions see above. Pandas fresh snapshot as of yesterday `1.1.0.dev0+1977.g1706d830d`.   

I'm pretty sure it's a question of my setup, so sorry for bothering you with it, but maybe someone has any idea or pointers of where to look for the reason of this behavior.