Use brown instead of white, in case people have black on white terminals
Add a summary for sphinx

Our current scores are

TOTAL DOCTEST SCORE for sympy: 34% (1984 of 5682)
TOTAL SPHINX SCORE for sympy: 22% (1293 of 5682)

which are not very good.
Tweak colors for coverage_doctest

Sphinx is now printed in gray, so it stands out less.  If the coverage is less
than 50%, the percent is printed in a brighter red.