Bump Python requirement mkdocstrings to 0.20.0

doc-requirements.txt

@@ -2,6 +2,6 @@ black==22.10.0
 doc8==1.0.0
 mkdocs==1.4.2
 mkdocs-material==8.5.11
-mkdocstrings==0.19.0
+mkdocstrings==0.20.0
 mkdocstrings-python==0.8.2
 recommonmark==0.7.1