Upgrade Python packages

[nlohmann]

• upgrade packages to fix security warning
• also bring mkdocs and other packages up to date

[coveralls]

Coverage remained the same at 100.0% when pulling 10b17d0 on upgrade_packages into 80dfb04 on develop.

[PDeets]

Thanks for making this change. However, I wanted to bring up that as a user of nlohnamm-json via vcpkg, I regularly get security warnings due to our security scanners finding the requirements.txt file and identifying what it believes are insecure dependencies of my source code. I'm just writing C++ code that uses nlohmann-json though, so I always have to dismiss these. However, it is a nuisance to deal with.

Do you think maybe we should add a step in the vcpkg portfile at https://github.com/microsoft/vcpkg/blob/master/ports/nlohmann-json/portfile.cmake to delete the docs folder after it downloads the source code for the tagged release so these dependencies don't show up in security scans.