--- a/mercurial/config.py	Sun Nov 10 07:30:14 2019 -0800
+++ b/mercurial/config.py	Fri Nov 08 11:19:20 2019 -0800
@@ -212,7 +212,7 @@
     def read(self, path, fp=None, sections=None, remap=None):
         if not fp:
             fp = util.posixfile(path, b'rb')
-        assert getattr(fp, 'mode', r'rb') == r'rb', (
+        assert getattr(fp, 'mode', 'rb') == 'rb', (
             b'config files must be opened in binary mode, got fp=%r mode=%r'
             % (fp, fp.mode,)
         )