tests/test-mq-qsave.t

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

  $ echo "[extensions]" >> $HGRCPATH
  $ echo "mq=" >> $HGRCPATH

  $ hg init

  $ echo 'base' > base
  $ hg ci -Ambase
  adding base

  $ hg qnew -mmqbase mqbase

  $ hg qsave
  $ hg qrestore 2
  restoring status: hg patches saved state