Dependencies: put upper limit markupsafe<2.1 (#5371)

The latest release `markupsafe==2.1` breaks `jinja<3.0` and that in
turn breaks `sphinx<4.0`. Upgrading to `sphinx~=4.0` brings its own
problems with a whole host of new warnings that cannot be easily fixed
and so instead we are putting an upper limit on `markupsafe`.

Cherry-pick: 139f7d2

setup.json

@@ -73,6 +73,7 @@
             "docutils==0.15.2",
             "pygments~=2.5",
             "pydata-sphinx-theme~=0.4.0",
+            "markupsafe<2.1",
             "sphinx~=3.2.1",
             "sphinxcontrib-details-directive~=0.1.0",
             "sphinx-panels~=0.5.0",