Bump Python requirement mkdocs to 1.4.0

docs-requirements.txt

 black==22.8.0
 doc8==0.11.1
-mkdocs==1.3.1
+mkdocs==1.4.0
 mkdocstrings==0.19.0
 mkdocstrings-python==0.7.1
 recommonmark==0.7.1