Add `pygments` to extras

Hello there :wave:

pygments is currently used to color the SQL output and it's only used when it's installed. That's great! On the other hand, since it doesn't explicitly require it, it leaves people depending on django-extensions to guess about which pygments are compatible with it.

My opinion is that it might be worth it, to add pygments to the extras_requires in setup.py .

Lmk what do you think.