Colorize pytest output

`pytest` is now explicitly told to colorize its output. This also
applies to the CI tests on GitHub.

Author: eerovaher

setup.cfg

@@ -33,7 +33,7 @@ astropy_header = true
 text_file_format = rst
 xfail_strict = true
 remote_data_strict = true
-addopts = --doctest-rst
+addopts = --color=yes --doctest-rst
 filterwarnings =
     error
 ##  These are temporary measures, all of these should be fixed: