chore: updates mypy config in pyproject.toml.

1. Use non-strict as starting point;
2. Apply python_version to 3.9.
3. Sets `follow_imports` to skip.

PiperOrigin-RevId: 763553672

pyproject.toml

@@ -157,7 +157,10 @@ asyncio_default_fixture_loop_scope = "function"
 asyncio_mode = "auto"
 
 [tool.mypy]
+python_version = "3.9"
 exclude = "tests/"
 plugins = ["pydantic.mypy"]
-strict = true
+# Start with non-strict mode, and swtich to strict mode later.
+# strict = true
 disable_error_code = ["import-not-found", "import-untyped", "unused-ignore"]
+follow_imports = "skip"