Apply suggestions from code review

Author: Borda

requirements/dev.txt

@@ -3,7 +3,8 @@ bump2version ==1.0.1
 # coverage >=4.5.4
 mkdocs-material ==9.5.*
 ipython[notebook] >8.0, <9.0
-mkdocstrings[python] ==0.27.*
+mkdocstrings[python] ==0.26.*; python_version < "3.9"
+mkdocstrings[python] ==0.27.*; python_version >= "3.9"
 mknotebooks ==0.8.*
 pytest >=5.3.2
 pytest-runner >=5.1