Unpin development version of Pygments.

Author: ZeroIntensity

Doc/requirements.txt

@@ -9,9 +9,7 @@
 # Keep this version in sync with ``Doc/conf.py``.
 sphinx<9.0.0
 
-# Temporary direct requirement, pending release of Pygments > 2.20.0
-# https://github.com/pygments/pygments/discussions/3145
-pygments @ https://github.com/pygments/pygments/archive/2cad2642058441b59782a6a18f03c98c42d081f1.tar.gz
+pygments>=2.21
 
 blurb