Fix wrong format in Python Poetry project file (#811)

Any extra packages must be marked as optional dependency.
diff --git a/extra/poetry/pyproject.toml b/extra/poetry/pyproject.toml
index c9858ce..3a41f22 100644
--- a/extra/poetry/pyproject.toml
+++ b/extra/poetry/pyproject.toml
@@ -19,7 +19,7 @@
 [tool.poetry.dependencies]
 python = ">=2.7"
 protobuf = ">=3.19"
-grpcio-tools = ">=1.46.0"
+grpcio-tools = {version=">=1.46.0", optional=true}
 
 [tool.poetry.dev-dependencies]