pinned version of jinja2 to fixed bug with versions

I found that the version of jinja2 must be pinnen to 3.0.0 in requirements.yml Otherwise I run into the error: AttributeError: module 'jinja2' has no attribute 'contextfilter'

The description on how to resolve this error can be found here: https://github.com/mkdocs/mkdocs/issues/2799