chore(bird): Improve code readability

roles/bird/filter_plugins/bird.py

@@ -89,7 +89,7 @@ class IPv4orIPv6(BaseModel):
     ipv6: list[int] = Field(ge=0, min_items=2, max_items=2)
 
 
-class IPFlag(str):
+class IPFlag:
     @classmethod
     def __get_validators__(cls):
         yield cls.validate
@@ -236,14 +236,8 @@ def str_of_condition(condition: Condition, ctx: bool) -> str:
                 return f"{min_v6} <= net.len && net.len <= {max_v6}"
 
         case NetMatch(matches=matches):
-            if ctx.ipv4:
-                networks = [
-                    m for m in matches if isinstance(m[0], IPv4Network)
-                ]
-            else:
-                networks = [
-                    m for m in matches if isinstance(m[0], IPv6Network)
-                ]
+            networkType = IPv4Network if ctx.ipv4 else IPv6Network
+            networks = [m for m in matches if isinstance(m[0], networkType)]
 
             return f"net ~ [ {', '.join([f'{network}{flag}' for (network, flag) in networks])} ]"