Fix whatsnew 3.15 RST heading that matches git conflict marker

The "Removed" heading underline was exactly 7 '=' characters, which
triggers the check-merge-conflict pre-commit hook as a false positive.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: gpshead

Doc/whatsnew/3.15.rst

@@ -1182,7 +1182,7 @@ Diego Russo in :gh:`140683` and :gh:`142305`.)
 
 
 Removed
-=======
+========
 
 ctypes
 ------