Pin Pygments to specific version

[justinmayer]

Recent updates have caused test failures, so perhaps we should pin this particular dependency.

[naturallymitchell]

Where'd the problem come from? If a recent change in Pygments caused it, how did devs there not catch it before pushing it downstream, at least without a 'change notice'; perhaps someone more familiar can dig into it? Is #1901 related (ping @nckx )? ..Pygments development looks surprisingly active.

[avaris]

Yes, #1901 is related. Problem is caused by Pygments producing different output than the previous one in the last two releases (2.1 and 2.1.1). This causes false negatives in pelican tests since the pre-generated outputs don't match the new ones. I don't know if generating consistent output is a goal in Pygments development, so this may or may not be an issue in their eyes.

[ingwinlu]

fixing the pygments version used for tests makes sense.