Support nbconvert 7.2.9

[Overboard]

My use case involves Jupyter notebooks that manipulate and show SVGs.

However, nbconvert<7.0.0 cannot process SVG output. See,
https://github.com/jupyter/nbconvert/blob/main/CHANGELOG.md#700
Specifically, jupyter/nbconvert#1837

With pipenv, mkdocs-jupyter is installing nbconvert v6.5.4

pipenv graph
mkdocs-jupyter==0.22.0
    ...
    "clipped for brevity"
    ...
  - nbconvert [required: >=6.2.0,<7.0.0, installed: 6.5.4]

Force installing nbconvert==7.2.9 of course results in #96

I'm assuming similar results for versions between 7.0.0 and 7.2.9 but in any case, support for the most recent would be ideal.

[josephmcasey]

I have a changeset that resolves this, and I am writing up the docs to open up a PR.