Flaky matplotlib tests
I'm frequently getting local test runs where nearly all tests/plotting tests will fail with something like

```

self = <pandas.tests.plotting.test_misc.TestSeriesPlots object at 0x12fc977b8>

    @pytest.mark.slow
    def test_autocorrelation_plot(self):
        from pandas.plotting import autocorrelation_plot
>       _check_plot_works(autocorrelation_plot, series=self.ts)

pandas/tests/plotting/test_misc.py:42: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 
pandas/tests/plotting/common.py:509: in _check_plot_works
    ret = f(**kwargs)
pandas/plotting/_misc.py:374: in autocorrelation_plot
    plot_backend = _get_plot_backend()
pandas/plotting/_core.py:630: in _get_plot_backend
    return importlib.import_module(backend_str)
/usr/local/Cellar/python/3.7.3/Frameworks/Python.framework/Versions/3.7/lib/python3.7/importlib/__init__.py:127: in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
<frozen importlib._bootstrap>:1006: in _gcd_import
    ???
<frozen importlib._bootstrap>:983: in _find_and_load
    ???
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

name = 'not_an_existing_module', import_ = <function _gcd_import at 0x10927ae18>

>   ???
E   ModuleNotFoundError: No module named 'not_an_existing_module'

<frozen importlib._bootstrap>:965: ModuleNotFoundError
```

Re-running with `--lf` consistently sees all of these pass.

OSX, PY37