policy: try and always have a bytes for module policy

debuginstall now runs cleanly in Python 3.

mercurial/policy.py

@@ -39,7 +39,7 b" if '__pypy__' in sys.builtin_module_name"
 # Our C extensions aren't yet compatible with Python 3. So use pure Python
 # on Python 3 for now.
 if sys.version_info[0] >= 3:
-    policy = 'py'
+    policy = b'py'
 
 # Environment variable can always force settings.
 policy = os.environ.get('HGMODULEPOLICY', policy)

setup.py

@@ -370,7 +370,7 b' class hgbuildpy(build_py):'
             modulepolicy = 'c'
         with open("mercurial/__modulepolicy__.py", "w") as f:
             f.write('# this file is autogenerated by setup.py\n')
-            f.write('modulepolicy = "%s"\n' % modulepolicy)
+            f.write('modulepolicy = b"%s"\n' % modulepolicy)
 
         build_py.run(self)
 

tests/test-check-py3-commands.t

@@ -11,7 +11,7 b' The full traceback is hidden to have a s'
   version
   warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
   debuginstall
-  TypeError: Can't convert 'bytes' object to str implicitly
+  no problems detected
 
   $ cat > included-hgrc <<EOF
   > [extensions]