TST: don't assert that matplotlib rejects shorthand hex colors
From 3.2 [it accepts them](https://matplotlib.org/users/prev_whats_new/whats_new_3.2.0.html#digit-and-4-digit-hex-colors).

[Test failure log](https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=954647) (from Debian's pandas 0.25, so includes other issues we don't now have - this one is DID NOT RAISE).
Thanks.  What version of matplotlib are you using? And you mentioned that log was from pandas 0.25; is the same issue present with 1.0.3?

With this diff from pandas master and matplotlib 3.1.2, it seems like a ValueError is raised when those colors are passed

```diff
diff --git a/pandas/tests/plotting/test_frame.py b/pandas/tests/plotting/test_frame.py
index 08b33ee547..194d29e291 100644
--- a/pandas/tests/plotting/test_frame.py
+++ b/pandas/tests/plotting/test_frame.py
@@ -2041,11 +2041,10 @@ class TestDataFramePlots(TestPlotBase):
         self._check_colors(ax.get_lines(), linecolors=custom_colors)
         tm.close()
 
-        with pytest.raises(ValueError):
-            # Color contains shorthand hex value results in ValueError
-            custom_colors = ["#F00", "#00F", "#FF0", "#000", "#FFF"]
-            # Forced show plot
-            _check_plot_works(df.plot, color=custom_colors)
+        # Color contains shorthand hex value results in ValueError
+        custom_colors = ["#F00", "#00F", "#FF0", "#000", "#FFF"]
+        # Forced show plot
+        _check_plot_works(df.plot, color=custom_colors)
 
     @pytest.mark.slow
     def test_dont_modify_colors(self):
```

That gives

```pytb
        # Color contains shorthand hex value results in ValueError
        custom_colors = ["#F00", "#00F", "#FF0", "#000", "#FFF"]
        # Forced show plot
>       _check_plot_works(df.plot, color=custom_colors)

pandas/tests/plotting/test_frame.py:2047:
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
pandas/tests/plotting/common.py:549: in _check_plot_works
    plt.savefig(path)
../../Envs/pandas-dev/lib/python3.7/site-packages/matplotlib/pyplot.py:722: in savefig
    res = fig.savefig(*args, **kwargs)
../../Envs/pandas-dev/lib/python3.7/site-packages/matplotlib/figure.py:2180: in savefig
    self.canvas.print_figure(fname, **kwargs)
../../Envs/pandas-dev/lib/python3.7/site-packages/matplotlib/backend_bases.py:2089: in print_figure
    **kwargs)
../../Envs/pandas-dev/lib/python3.7/site-packages/matplotlib/backends/backend_agg.py:527: in print_png
    FigureCanvasAgg.draw(self)
../../Envs/pandas-dev/lib/python3.7/site-packages/matplotlib/backends/backend_agg.py:388: in draw
    self.figure.draw(self.renderer)
../../Envs/pandas-dev/lib/python3.7/site-packages/matplotlib/artist.py:38: in draw_wrapper
    return draw(artist, renderer, *args, **kwargs)
../../Envs/pandas-dev/lib/python3.7/site-packages/matplotlib/figure.py:1709: in draw
    renderer, self, artists, self.suppressComposite)
../../Envs/pandas-dev/lib/python3.7/site-packages/matplotlib/image.py:135: in _draw_list_compositing_images
    a.draw(renderer)
../../Envs/pandas-dev/lib/python3.7/site-packages/matplotlib/artist.py:38: in draw_wrapper
    return draw(artist, renderer, *args, **kwargs)
../../Envs/pandas-dev/lib/python3.7/site-packages/matplotlib/axes/_base.py:2647: in draw
    mimage._draw_list_compositing_images(renderer, self, artists)
../../Envs/pandas-dev/lib/python3.7/site-packages/matplotlib/image.py:135: in _draw_list_compositing_images
    a.draw(renderer)
../../Envs/pandas-dev/lib/python3.7/site-packages/matplotlib/artist.py:38: in draw_wrapper
    return draw(artist, renderer, *args, **kwargs)
../../Envs/pandas-dev/lib/python3.7/site-packages/matplotlib/lines.py:783: in draw
    lc_rgba = mcolors.to_rgba(self._color, self._alpha)
../../Envs/pandas-dev/lib/python3.7/site-packages/matplotlib/colors.py:177: in to_rgba
    rgba = _to_rgba_no_colorcycle(c, alpha)
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

c = '#F00', alpha = None

    def _to_rgba_no_colorcycle(c, alpha=None):
        """Convert *c* to an RGBA color, with no support for color-cycle syntax.

        If *alpha* is not ``None``, it forces the alpha value, except if *c* is
        ``"none"`` (case-insensitive), which always maps to ``(0, 0, 0, 0)``.
        """
        orig_c = c
        if c is np.ma.masked:
            return (0., 0., 0., 0.)
        if isinstance(c, str):
            if c.lower() == "none":
                return (0., 0., 0., 0.)
            # Named color.
            try:
                # This may turn c into a non-string, so we check again below.
                c = _colors_full_map[c]
            except KeyError:
                try:
                    c = _colors_full_map[c.lower()]
                except KeyError:
                    pass
                else:
                    if len(orig_c) == 1:
                        cbook.warn_deprecated(
                            "3.1", message="Support for uppercase "
                            "single-letter colors is deprecated since Matplotlib "
                            "%(since)s and will be removed %(removal)s; please "
                            "use lowercase instead.")
        if isinstance(c, str):
            # hex color with no alpha.
            match = re.match(r"\A#[a-fA-F0-9]{6}\Z", c)
            if match:
                return (tuple(int(n, 16) / 255
                              for n in [c[1:3], c[3:5], c[5:7]])
                        + (alpha if alpha is not None else 1.,))
            # hex color with alpha.
            match = re.match(r"\A#[a-fA-F0-9]{8}\Z", c)
            if match:
                color = [int(n, 16) / 255
                         for n in [c[1:3], c[3:5], c[5:7], c[7:9]]]
                if alpha is not None:
                    color[-1] = alpha
                return tuple(color)
            # string gray.
            try:
                return (float(c),) * 3 + (alpha if alpha is not None else 1.,)
            except ValueError:
                pass
>           raise ValueError("Invalid RGBA argument: {!r}".format(orig_c))
E           ValueError: Invalid RGBA argument: '#F00'

../../Envs/pandas-dev/lib/python3.7/site-packages/matplotlib/colors.py:233: ValueError
```


but I have no idea why we're asserting that matplotlib raises a ValueError. Debian unstable (matplotlib 3.2.1, numpy 1.17.4, Python 3.8.2).  It occurs in pandas 1.0.3 with Debian patches; I haven't tried master.

The matplotlib release notes say this changed in 3.2; I removed the "must fail" tests rather than replacing them with "must work" to keep compatibility with older versions.

This test was added by #11293. Thanks for tracking down those links. Agreed that these can be removed.