Update deps & add generate_lib task

- Remove plugin dependency on protobuf since it's no longer required.
- Update poethepoet to for better pyproject toml syntax support
- Add handy generate_lib poe task for maintaining generated libs

tests/inputs/oneof/test_oneof.py

@@ -6,10 +6,10 @@ from tests.util import get_test_case_json_data
 def test_which_count():
     message = Test()
     message.from_json(get_test_case_json_data("oneof")[0])
-    assert betterproto.which_one_of(message, "foo") == ("count", 100)
+    assert betterproto.which_one_of(message, "foo") == ("pitied", 100)
 
 
 def test_which_name():
     message = Test()
     message.from_json(get_test_case_json_data("oneof", "oneof_name.json")[0])
-    assert betterproto.which_one_of(message, "foo") == ("name", "foobar")
+    assert betterproto.which_one_of(message, "foo") == ("pitier", "Mr. T")