test: Restrict tf dependency protobuf to ABI compatible releases (#1869)

* protobuf v3.20.1 is the last release to support ABI compatibility
with tensorflow pre tensorflow 2.9.1. The next release of protobuf,
v4.21.0, breaks ABI compatibility and so requires an upper bound of
protobuf v3.20.1 for older tensorflow releases to not break. tensorflow
released a series of patch releases in response to protobuf v4.21.0:
v2.6.5, v2.7.3, v2.8.2, v2.9.1.
   - c.f. tensorflow/tensorflow#56077

Author: matthewfeickert

tests/constraints.txt

@@ -12,6 +12,7 @@ uproot==4.1.1
 iminuit==2.4.0
 # tensorflow
 tensorflow==2.3.1  # tensorflow-probability v0.11.0 requires tensorflow>=2.3
+protobuf<=3.20.1  # protobuf v4.21.0 breaks ABI compatability
 tensorflow-probability==0.11.0  # c.f. PR #1657
 # torch
 torch==1.10.0