Bump Python requirement mkdocs-material to 9.0.6

doc-requirements.txt

 black==22.10.0
 doc8==1.0.0
 mkdocs==1.4.2
-mkdocs-material==8.5.11
+mkdocs-material==9.0.6
 mkdocstrings==0.19.0
 mkdocstrings-python==0.8.3
 recommonmark==0.7.1