py3: add pycompat.open and replace open() calls

open() requires mode argument as unicodes on Python 3. This patch introduces
pycompat.open() which is inserted to files using transformer and replaces
builtins.open() calls.

mercurial/__init__.py

@@ -280,7 +280,7 b' if sys.version_info[0] >= 3:'
                     continue
                 r, c = t.start
                 l = (b'; from mercurial.pycompat import '
-                     b'delattr, getattr, hasattr, setattr, xrange\n')
+                     b'delattr, getattr, hasattr, setattr, xrange, open\n')
                 for u in tokenize.tokenize(io.BytesIO(l).readline):
                     if u.type in (tokenize.ENCODING, token.ENDMARKER):
                         continue
@@ -327,7 +327,7 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\x06'
+    BYTECODEHEADER = b'HG\x00\x07'
 
     class hgloader(importlib.machinery.SourceFileLoader):
         """Custom module loader that transforms source code.

mercurial/pycompat.py

@@ -96,6 +96,9 b' if ispy3:'
     setattr = _wrapattrfunc(builtins.setattr)
     xrange = builtins.range
 
+    def open(name, mode='r', buffering=-1):
+        return builtins.open(name, sysstr(mode), buffering)
+
     # getopt.getopt() on Python 3 deals with unicodes internally so we cannot
     # pass bytes there. Passing unicodes will result in unicodes as return
     # values which we need to convert again to bytes.