requirements: force newer Pygments version for TOML lexer

Problem: Pygments only added a TOML lexer in 2.4. So older versions do
not support syntax highlighting of TOML.

Solution: Pin the requirements to a newer version.

requirements.txt

@@ -1,3 +1,4 @@
+Pygments>=2.4
 Sphinx
 sphinxcontrib-spelling
 sphinx-rtd-theme