Don't allow pyzmq 21.0.0 on AppVeyor.

This version is broken on Windows:
zeromq/pyzmq#1470.

Author: QuLogic

.appveyor.yml

@@ -18,6 +18,7 @@ environment:
     PYTHONIOENCODING: UTF-8
     PYTEST_ARGS: -raR --numprocesses=auto --timeout=300 --durations=25
                  --cov-report= --cov=lib --log-level=DEBUG
+    PINNEDVERS: "pyzmq!=21.0.0"
 
   matrix:
     # In theory we could use a single CONDA_INSTALL_LOCN because we construct