Disable colorization if terminal does not support colors

[adamchainz]

A failing test in a project I'm working on with pytest-icdiff failed on CI where there is no terminal color support, and the output contains all the ANSI colorization characters which makes it much harder to read:

__________________________ test_foo ___________________________
[gw5] linux -- Python 3.7.3 /home/ciuser/.../python3
tests/test_foo.py:112: in test_foo
    assert data == {
E   AssertionError: assert equals failed
E     �[m�[1;31m{'id': '00000000-0000-0000-0000-�[m  �[1;32m{�[m                                
E     �[m�[1;31m000000000002', 'created': '2020-�[m                                   
...                       
E     �[m                                  �[1;32m  'created': '2020-09-24T00:00:0�[m 
E     �[m                                  �[1;32m0Z',�[m                             
...

Colorization should only be done if sys.stdout.isatty() returns True - for example see my utility https://github.com/adamchainz/lifelogger/blob/master/lifelogger/utils.py#L10 .

Writing a PR now.