Cap the Pygments dependency at < 2.6.
Cap the Pygments dependency at < 2.6.

Pygments dropped Python 2 support with Pygments 2.6. While it might
still currently work on 2.6, we don't want to risk future versions
breaking.

This change caps our dependency for Review Board 3.0 at < 2.6.

Testing Done:
Uninstalled Pygments and re-ran `./setup.py develop`. Verified it
installed Pygments 2.5.2 (the latest Python 2.x release).

Reviewed at https://reviews.reviewboard.org/r/10983/

reviewboard/reviewboard,chipx86/reviewboard,chipx86/reviewboard,chipx86/reviewboard,chipx86/reviewboard,reviewboard/reviewboard,reviewboard/reviewboard,reviewboard/reviewboard