Fix bytes regex for Python 3.

Closes gh-4602

IPython/parallel/apps/baseapp.py

@@ -272,5 +272,5 b' class BaseParallelApplication(BaseIPythonApplication):'
                     " Making the likely assumption that it is."%pid
                 )
                 return True
-            pids = list(map(int, re.findall(r'^\W*\d+', output, re.MULTILINE)))
+            pids = list(map(int, re.findall(br'^\W*\d+', output, re.MULTILINE)))
             return pid in pids