Add comment explaining motivation for the compatibility shim.

src/towncrier/_writer.py

@@ -15,7 +15,7 @@
 
 
 if sys.version_info < (3, 10):
-
+    # Compatibility shim for newline parameter to write_text, added in 3.10
     def _newline_write_text(path: Path, content: str, **kwargs: Any) -> None:
         with path.open("w", **kwargs) as strm:  # pragma: no branch
             strm.write(content)