Relax version on pygments

[dschwoerer]

Allows to have the most recent version of pygments installed, if installing FORD.

https://pypi.org/project/Pygments/ is at 2.15.1

• [x] Test runs without errors
• [x] Creating EMC3 docs works without any errors
• [x] I did check the output, and could not notice any regressions

[ZedThree]

Thanks @dschwoerer!