Revert part of 415bc766401122 which breaks packaging with py2

Index: modulegraph/util.py
--- modulegraph/util.py.orig
+++ modulegraph/util.py
@@ -105,7 +105,7 @@ def imp_walk(name):
     raise ImportError("No module named %s" % (name,))
 
 
-cookie_re = re.compile(rb"coding[:=]\s*([-\w.]+)")
+cookie_re = re.compile(br"coding[:=]\s*([-\w.]+)")
 if sys.version_info[0] == 2:
     default_encoding = "ascii"
 else: