py3: add b'' prefix to make the regex bytes

# skip-blame because just b'' prefixes

Differential Revision: https://phab.mercurial-scm.org/D3705

tests/run-tests.py

@@ -2646,7 +2646,7 b' class TestRunner(object):'
                 expanded_args.append(arg)
         args = expanded_args
 
-        testcasepattern = re.compile(r'([\w-]+\.t|py)(#([^\s]+))')
+        testcasepattern = re.compile(br'([\w-]+\.t|py)(#([^\s]+))')
         tests = []
         for t in args:
             case = None