encoding: define local identify functions with explicit type comments

This removes some obfuscation as far as pytype is concerned and
corrects many bogus type errors.

Differential Revision: https://phab.mercurial-scm.org/D7267

mercurial/encoding.py

@@ -241,8 +241,15 b' if pycompat.ispy3:'
     strfromlocal = unifromlocal
     strmethod = unimethod
 else:
-    strtolocal = pycompat.identity
-    strfromlocal = pycompat.identity
+
+    def strtolocal(s):
+        # type: (str) -> bytes
+        return s
+
+    def strfromlocal(s):
+        # type: (bytes) -> str
+        return s
+
     strmethod = pycompat.identity
 
 if not _nativeenviron: