py3: add pycompat.unicode and add it to importer

On python 3, builtins.unicode does not exist.

mercurial/__init__.py

@@ -283,7 +283,8 b' if sys.version_info[0] >= 3:'
                     continue
                 r, c = t.start
                 l = (b'; from mercurial.pycompat import '
-                     b'delattr, getattr, hasattr, setattr, xrange, open\n')
+                     b'delattr, getattr, hasattr, setattr, xrange, '
+                     b'open, unicode\n')
                 for u in tokenize.tokenize(io.BytesIO(l).readline):
                     if u.type in (tokenize.ENCODING, token.ENDMARKER):
                         continue
@@ -323,7 +324,7 b' if sys.version_info[0] >= 3:'
     # ``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\x09'
+    BYTECODEHEADER = b'HG\x00\x0a'
 
     class hgloader(importlib.machinery.SourceFileLoader):
         """Custom module loader that transforms source code.

mercurial/pycompat.py

@@ -174,6 +174,7 b' if ispy3:'
     hasattr = _wrapattrfunc(builtins.hasattr)
     setattr = _wrapattrfunc(builtins.setattr)
     xrange = builtins.range
+    unicode = str
 
     def open(name, mode='r', buffering=-1):
         return builtins.open(name, sysstr(mode), buffering)