update already-backported search pattern

catches PRs merged before splitting the 3.x branch

tools/backport_pr.py

@@ -107,7 +107,7 b" def backport_pr(branch, num, project='ipython/ipython'):"
 
     return 0
 
-backport_re = re.compile(r"[Bb]ackport.*?(\d+)")
+backport_re = re.compile(r"(?:[Bb]ackport|[Mm]erge).*#(\d+)")
 
 def already_backported(branch, since_tag=None):
     """return set of PRs that have been backported already"""