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

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.

--- a/mercurial/__init__.py	Fri Aug 05 13:56:10 2016 +0200
+++ b/mercurial/__init__.py	Fri Mar 03 13:04:32 2017 +0530
@@ -280,7 +280,7 @@
                     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 @@
     # ``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.

--- a/mercurial/pycompat.py	Fri Aug 05 13:56:10 2016 +0200
+++ b/mercurial/pycompat.py	Fri Mar 03 13:04:32 2017 +0530
@@ -96,6 +96,9 @@
     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.