add python extra for mkdocstrings (#96)

requirements-dev.txt

@@ -6,7 +6,7 @@ flake8>=3.7.8
 mike
 mkdocs>=1.2.2
 mkdocs-material>=7.2.6
-mkdocstrings>=0.15.2
+mkdocstrings[python]>=0.19.0
 mypy>=0.782
 pip>=19.2.3
 pytest-runner>=5.1