Pin Jinja2 to avoid cutting out a lot of not that old Sphinx versions

setup.py

@@ -28,6 +28,12 @@
         # "really" support 4.x. May as well not artificially limit folks for
         # now.
         "sphinx>=1.8",
+        # Jinja2 3.0 throws deprecation warnings on all sphinx<4.x, and
+        # 3.1 then removes what was deprecated. Rather than cut out support for
+        # all Sphinx versions published earlier than mid 2021, I'll instead
+        # eschew newer Jinjas, and hope nobody using my Sphinx tools seriously
+        # _need_ whatever came out in Jinja 3...
+        "Jinja2<3",
         # Continuing to pin an old semantic_version until I have time to update
         # and finish the branch I made for
         # https://github.com/bitprophet/releases/pull/86#issuecomment-580037996