Convert pytest warnings into errors (#424)

Just a ratchet to ensure we don't inadvertently miss warnings in CI
output.

Similar to connect-python: connectrpc/connect-python#118

Author: stefanvanburen

pyproject.toml

@@ -109,6 +109,9 @@ ban-relative-imports = "all"
 strict = true
 # restrict testpaths to speed up test discovery
 testpaths = ["test"]
+# Turn all warnings into errors,
+# except DeprecationWarnings (which we knowingly tolerate due to using old `protobuf` APIs).
+filterwarnings = ["error", "ignore::DeprecationWarning"]
 
 [tool.mypy]
 mypy_path = "gen"