fix dataclasses warnings

e621.py

@@ -20,7 +20,7 @@ class E621PostFile:
     ext: str
     size: int
     md5: str
-    url: str
+    url: Optional[str]
 
 
 @dataclass_json
@@ -40,7 +40,7 @@ class E621PostFlags:
 class E621PostPreview:
     width: int
     height: int
-    url: str
+    url: Optional[str]
 
 
 @dataclass_json
@@ -73,7 +73,7 @@ class E621PostSample:
     has: bool
     height: int
     width: int
-    url: str
+    url: Optional[str]
     # alternates: E621PostAlternates
 
 
@@ -121,7 +121,7 @@ class E621Post:
     sources: List[str]
     pools: List[int]
     relationships: E621PostRelationships
-    approver_id: None
+    approver_id: Optional[int]
     uploader_id: int
     description: str
     comment_count: int