fix: relax protobuf dependency version requirement (#381)

We want to use this in a project that requires `protobuf==6.31.1`.
Version history indicates upgrading a2a-python to that version is not
possible at the moment. Therefore, relax the required version to just be
`>=5.29.5` (instead of `==`).

This does print a warning when using a2a-python with a (major) newer
version, but my tests show that it works just fine. Better to live with
the warning than not being able to use a2a-python at all.

This does not update the buf generation config as this would upgrade to
newer proto requirements (and generate protos for newer versions).

Author: timn

pyproject.toml

@@ -14,7 +14,7 @@ dependencies = [
   "pydantic>=2.11.3",
   "sse-starlette",
   "starlette",
-  "protobuf==5.29.5",
+  "protobuf>=5.29.5",
   "google-api-core>=1.26.0",
 ]