add xeus-cling to external tests
it should exercise the pinning issue
url: https://github.com/binder-examples/requirements
ref: test
verify: python -c 'import matplotlib'
+# Test that custom channels and downgrades don't
+# get blocked by pinning
+- name: Xeus-Cling
+ url: https://github.com/QuantStack/xeus-cling
+ ref: 0.4.5
+ verify: jupyter kernelspec list
