Pythonize input_type name in py_input_message (#436)

Co-authored-by: konstantin <konstantin.klein@hochfrequenz.de>
Co-authored-by: James Hilton-Balfe <gobot1234yt@gmail.com>
Fixes https://github.com/danielgtaylor/python-betterproto/issues/427
Fixes https://github.com/danielgtaylor/python-betterproto/issues/438

tests/inputs/service_uppercase/service.proto

@@ -0,0 +1,16 @@
+syntax = "proto3";
+
+package service_uppercase;
+
+message DoTHINGRequest {
+  string name = 1;
+  repeated string comments = 2;
+}
+
+message DoTHINGResponse {
+  repeated string names = 1;
+}
+
+service Test {
+  rpc DoThing (DoTHINGRequest) returns (DoTHINGResponse);
+}