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