diff --git a/mercurial/__init__.py b/mercurial/__init__.py
--- a/mercurial/__init__.py
+++ b/mercurial/__init__.py
@@ -207,6 +207,13 @@
                         if argidx is not None:
                             _ensureunicode(argidx)
 
+                # Bare open call (not an attribute on something else), the
+                # second argument (mode) must be a string, not bytes
+                elif fn == 'open' and not _isop(i - 1, '.'):
+                    arg1idx = _findargnofcall(1)
+                    if arg1idx is not None:
+                        _ensureunicode(arg1idx)
+
                 # It changes iteritems/values to items/values as they are not
                 # present in Python 3 world.
                 elif fn in ('iteritems', 'itervalues'):
@@ -220,7 +227,7 @@
     # ``replacetoken`` or any mechanism that changes semantics of module
     # loading is changed. Otherwise cached bytecode may get loaded without
     # the new transformation mechanisms applied.
-    BYTECODEHEADER = b'HG\x00\x0a'
+    BYTECODEHEADER = b'HG\x00\x0b'
 
     class hgloader(importlib.machinery.SourceFileLoader):
         """Custom module loader that transforms source code.