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).