requirements.txt: Pin protobuf

TypeError: Descriptors cannot be created directly.
If this call came from a _pb2.py file, your generated code is out of date and must be regenerated with protoc >= 3.19.0.
If you cannot immediately regenerate your protos, some other possible workarounds are:
 1. Downgrade the protobuf package to 3.20.x or lower.

Signed-off-by: Frode Nordahl <[email protected]>

Author: fnordahl

requirements.txt

@@ -62,3 +62,5 @@ git+https://github.com/openstack-charmers/zaza#egg=zaza
 # * https://github.com/openstack-charmers/zaza/issues/421
 # * https://mail.python.org/pipermail/cryptography-dev/2021-January/001003.html
 cryptography<3.4
+
+protobuf<3.21.0