Bump Python requirement mkdocs-material to 9.1.8

doc-requirements.txt

 black==23.1.0
 doc8==1.1.1
 mkdocs==1.4.2
-mkdocs-material==9.1.3
+mkdocs-material==9.1.8
 mkdocstrings==0.20.0
 mkdocstrings-python==0.9.0
 recommonmark==0.7.1