pin jinja2<3.1.* for docs (#5674)

Author: pmeier

docs/requirements.txt

@@ -3,4 +3,6 @@ numpy
 sphinx-copybutton>=0.3.1
 sphinx-gallery>=0.9.0
 sphinx==3.5.4
+# This pin is only needed for sphinx<4.0.2. See https://github.com/pytorch/vision/issues/5673 for details
+Jinja2<3.1.*
 -e git+https://github.com/pytorch/pytorch_sphinx_theme.git#egg=pytorch_sphinx_theme