Fix most mypy warnings

betterproto/plugin.py

@@ -6,10 +6,10 @@ import re
 import stringcase
 import sys
 import textwrap
-from typing import List
+from typing import List, Union
+import betterproto
 from betterproto.casing import safe_snake_case
 from betterproto.compile.importing import get_ref_type
-import betterproto
 
 try:
     # betterproto[compiler] specific dependencies
@@ -58,8 +58,8 @@ def py_type(
         raise NotImplementedError(f"Unknown type {descriptor.type}")
 
 
-def get_py_zero(type_num: int) -> str:
-    zero = 0
+def get_py_zero(type_num: int) -> Union[str, float]:
+    zero: Union[str, float] = 0
     if type_num in []:
         zero = 0.0
     elif type_num == 8: