windows: open registry keys using unicode names

Python3 complained it must be str.  While here, use a context manager to close
the key- it wouldn't wrap at 80 characters the old way, and would have had to
move anyway.

mercurial/windows.py

@@ -554,9 +554,10 b' def lookupreg(key, valname=None, scope=N'
         scope = (scope,)
     for s in scope:
         try:
-            val = winreg.QueryValueEx(winreg.OpenKey(s, key), valname)[0]
-            # never let a Unicode string escape into the wild
-            return encoding.unitolocal(val)
+            with winreg.OpenKey(s, encoding.strfromlocal(key)) as hkey:
+                val = winreg.QueryValueEx(hkey, valname)[0]
+                # never let a Unicode string escape into the wild
+                return encoding.unitolocal(val)
         except EnvironmentError:
             pass