Remove Jinja2 version pinning

It was introduced in response to #821,
but the underlying issue seems resolved (no import error during linkcheck).
An upgrade is necessary because of a vulnerability report in Jinja2, which was
patched in version 3.1.3.
Vulnerability report: https://github.com/datalad-handbook/book/security/dependabot/1

requirements.txt

@@ -5,7 +5,7 @@ chardet
 docutils
 idna
 imagesize
-Jinja2 < 3.1
+Jinja2
 MarkupSafe
 packaging
 Pygments