build_emoji: Guard mutable default with read-only type.

Signed-off-by: Anders Kaseorg <anders@zulip.com>

tools/setup/emoji/build_emoji

@@ -5,7 +5,7 @@
 import os
 import shutil
 import sys
-from typing import Any, Dict, Iterator, List, Optional
+from typing import Any, Dict, Iterator, List, Optional, Sequence
 
 import orjson
 
@@ -127,13 +127,13 @@ def percent(f: float) -> str:
     return f"{f * 100:0.3f}%"
 
 
-def get_square_size(emoji_data: List[Dict[str, Any]]) -> int:
+def get_square_size(emoji_data: Sequence[Dict[str, Any]]) -> int:
     """
     Spritesheets are usually NxN squares, and we have to
     infer N from the sheet_x/sheet_y values of emojis.
     """
 
-    def get_offsets(emoji_data: List[Dict[str, Any]]) -> Iterator[int]:
+    def get_offsets(emoji_data: Sequence[Dict[str, Any]]) -> Iterator[int]:
         for emoji_dict in emoji_data:
             yield emoji_dict["sheet_x"]
             yield emoji_dict["sheet_y"]
@@ -151,7 +151,7 @@ def generate_sprite_css_files(
     emoji_data: List[Dict[str, Any]],
     emojiset: str,
     alt_name: str,
-    fallback_emoji_data: List[Dict[str, Any]],
+    fallback_emoji_data: Sequence[Dict[str, Any]],
 ) -> None:
     """
     Spritesheets are usually NxN squares.
@@ -281,7 +281,7 @@ def setup_emoji_farms(cache_path: str, emoji_data: List[Dict[str, Any]]) -> None
         emojiset: str,
         emoji_data: List[Dict[str, Any]],
         alt_name: Optional[str] = None,
-        fallback_emoji_data: List[Dict[str, Any]] = list(),
+        fallback_emoji_data: Sequence[Dict[str, Any]] = [],
     ) -> None:
         # `alt_name` is an optional parameter that we use to avoid duplicating below
         # code. It is only used while setting up google-blob emoji set as it is just