Fix tests on main (#2423)
A new protobuf release on pypi broke our tests.
This is the same version that pip installs:
```console
$ pip install hashb_foxglove_protocolbuffers_python==25.3.0.1.20240226043130+465630478360 --extra-index-url https://buf.build/gen/python
Looking in indexes: https://pypi.org/simple, https://buf.build/gen/python
Collecting hashb_foxglove_protocolbuffers_python==25.3.0.1.20240226043130+465630478360
Downloading https://buf.build/gen/python/hashb-foxglove-protocolbuffers-python/hashb_foxglove_protocolbuffers_python-25.3.0.1.20240226043130%2B465630478360-py3-none-any.whl
- 34.1 kB 1.9 MB/s 0:00:00
Collecting protobuf (from hashb_foxglove_protocolbuffers_python==25.3.0.1.20240226043130+465630478360)
Downloading protobuf-5.26.0-cp37-abi3-manylinux2014_x86_64.whl.metadata (592 bytes)
Downloading protobuf-5.26.0-cp37-abi3-manylinux2014_x86_64.whl (302 kB)
━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ 302.8/302.8 kB 2.8 MB/s eta 0:00:00
Installing collected packages: protobuf, hashb_foxglove_protocolbuffers_python
Successfully installed hashb_foxglove_protocolbuffers_python-25.3.0.1.20240226043130+465630478360 protobuf-5.26.0
```
I added a constraints file for future releases of protobuf.
