Use 'utf-8' for consistency with other modules.

src/towncrier/_writer.py

@@ -40,7 +40,7 @@ def append_to_newsfile(
     # Leave newlines alone. This probably leads to inconsistent newlines,
     # because we've loaded existing content with universal newlines, but that's
     # the original behavior.
-    with news_file.open("w", encoding="utf8", newline="") as f:
+    with news_file.open("w", encoding="utf-8", newline="") as f:
         if header:
             f.write(header)
         # If there is no previous body that means we're writing a brand new news file.