Merge pull request #17382 from e-kayrakli/sphinxcontrib-0.0.20

Bump up sphinxcontrib version to 0.0.20

The new version has the new keywords we are adding in Chapel 1.24, as well as
the support for the new `:` operator.

[Reviewed by @lydia-duncan]

Author: e-kayrakli

third-party/chpl-venv/chpldoc-requirements.txt

@@ -5,5 +5,5 @@ Pygments==2.7.1
 Sphinx==3.2.1
 docutils==0.16
 sphinx-rtd-theme==0.5.0
-sphinxcontrib-chapeldomain==0.0.19
+sphinxcontrib-chapeldomain==0.0.20
 babel==2.8.0