py3: manually import pycompat.open into files that need it We want to eliminate the source transformer. Currently it inserts a `from mercurial.pycompat import ...` at the top of files to alias some builtins. This commit replaces the implicit import of `open` with an explicit import on files that need it and changes the source transformer to no longer import `open`. As part of this, we needed to store an explicit local for `open` in the Python 2 code path in `pycompat` so the import works. (Builtins that are automatically in scope cannot be imported.) Differential Revision: https://phab.mercurial-scm.org/D7005

--- a/mercurial/pycompat.py	Sun Oct 06 13:17:19 2019 -0400
+++ b/mercurial/pycompat.py	Sun Oct 06 13:28:56 2019 -0400
@@ -354,6 +354,7 @@
     sysstr = identity
     strurl = identity
     bytesurl = identity
+    open = open
 
     # this can't be parsed on Python 3
     exec(b'def raisewithtb(exc, tb):\n' b'    raise exc, None, tb\n')