[FW][FIX] requirements.txt: pin Jinja2 < 3.1

[fw-bot]

The latest versions of Jinja2 are no longer compatible with the latest
version of Sphinx 3 (3.5.4).

task-2828982

Forward-Port-Of: #1991

[robodoo]

Pull request status dashboard.

[fw-bot]

Ping @AntoineVDV cherrypicking of pull request #1991 failed.

stderr:

18:51:15.379987 git.c:344               trace: built-in: git cherry-pick 39d67b98ae0ce95fe42bbd927436dd1792c4829f
error: could not apply 39d67b98... [FIX] requirements.txt: pin Jinja2 < 3.1
hint: after resolving the conflicts, mark the corrected paths
hint: with 'git add <paths>' or 'git rm <paths>'
hint: and commit the result with 'git commit'
----------
status:

Either perform the forward-port manually (and push to this branch, proceeding as usual) or close this PR (maybe?).

In the former case, you may want to edit this PR message as well.

[AntoineVDV]

@robodoo r+