Bump Python requirement mkdocs-material to 9.0.15

docs-requirements.txt

 black==22.12.0
 doc8==1.1.1
 mkdocs==1.4.2
-mkdocs-material==9.0.10
+mkdocs-material==9.0.15
 mkdocstrings==0.20.0
 mkdocstrings-python==0.8.3
 recommonmark==0.7.1