requirements.txt: Remove sphinx version pin

Encountered issues have been fixed so we can remove the pinning of sphinx.

requirements.txt

 # Add here external Python modules dependencies, one per line. Module names
 # should match https://pypi.python.org/pypi names. For the full spec or
 # dependency lines, see https://pip.readthedocs.org/en/1.1/requirements.html
-sphinx < 4.4.0  # version 4.4.0 adds this kind of warning: "WARNING: hardcoded link 'https://archive.softwareheritage.org/api/1/vault/directory/doc/' could be replaced by an extlink (try using ':swh_web_api:`vault/directory/doc/`' instead)", which breaks the CI since we fail on warnings
+sphinx
 sphinxcontrib-httpdomain >= 1.8.0
 sphinxcontrib-images
 sphinxcontrib-programoutput