test: Add constraint to protobuf for lower bound tests (#2117)

* Add constraint of 'protobuf<4.21.0' for tensorflow==2.7.0 to work in the
  minimum supported dependencies workflow. Without this constraint on protobuf
  a 'TypeError: Descriptors cannot not be created directly' will occur:

   E   TypeError: Descriptors cannot not be created directly.
   E   If this call came from a _pb2.py file, your generated code is out of date and must be regenerated with protoc 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.
   E   If you cannot immediately regenerate your protos, some other possible workarounds are:
   E    1. Downgrade the protobuf package to 3.20.x or lower.
   E    2. Set PROTOCOL_BUFFERS_PYTHON_IMPLEMENTATION=python (but this will use pure-Python parsing and will be much slower).

Author: matthewfeickert

tests/constraints.txt

@@ -14,6 +14,7 @@ iminuit==2.7.0  # c.f. PR #1895
 # tensorflow
 tensorflow==2.7.0  # c.f. PR #1962
 tensorflow-probability==0.11.0  # c.f. PR #1657
+protobuf<4.21.0  # c.f. PR #2117
 # torch
 torch==1.10.0
 # jax