Bump Python requirement mkdocs to 1.5.3

docs-requirements.txt

 black==23.7.0
 doc8==1.1.1
-mkdocs==1.5.2
+mkdocs==1.5.3
 mkdocs-material==9.1.21
 mkdocstrings==0.22.0
 mkdocstrings-python==1.3.0