Upgrade dependencies

[ebouchut]

This PR upgrades the project's (Python) dependencies.

For the record, here is how I upgraded all the project's (direct (i.e., top-level)) dependencies declared in requirements.in:

pip-compile -v -U requirements.in

This command identifies the most up-to-date dependencies that are compatible with each other and updates the locked dependencies file (requirements.txt) with the exhaustive list of direct and indirect dependencies.

Next, anyone using mkdocs to preview LoopDocs locally should update the dependencies:

pip install -r requirements.txt

[ebouchut]

👍 LGTM