Fix `make docs`

Building the documentation has been giving an import error due to incompatibility between old Sphinx versions and new Jinja2 version. The problem is discussed in this Sphinx ticket.

I think there are two possible solutions: one is to restrict the Jinja2 version (which I have implemented in this MR now), and the other is to start using a newer version of Sphinx. The latter may be preferable, but would require checking whether there are no changes that break our docs. I went for the easy option, but if anyone wants to look into a better fix feel welcome.

I am now randomly fixing this in netsquid-magic, but the issue seems to persist in almost all netsquid snippets. If we have an accepted solution we should roll it out in all of them. (I looked at a view at random and saw that in netsquid-netconf we dropped the upper bound on sphinx, so presumably that is an effective fix).

As this change doesn't actually change the deployed code, I think we can merge directly and silently into master without bumping the version.