tests/test-absorb-phase.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

  $ cat >> $HGRCPATH << EOF
  > [extensions]
  > absorb=
  > drawdag=$TESTDIR/drawdag.py
  > EOF

  $ hg init
  $ hg debugdrawdag <<'EOS'
  > C
  > |
  > B
  > |
  > A
  > EOS

  $ hg phase -r A --public -q
  $ hg phase -r C --secret --force -q

  $ hg update C -q
  $ printf B1 > B

  $ hg absorb -aq

  $ hg log -G -T '{desc} {phase}'
  @  C secret
  |
  o  B draft
  |
  o  A public