tests/test-username-newline.t

py3: a second argument to open can't be bytes

This fixes open(filename, 'r'), open(filename, 'w'), etc. calls. In Python
3, that second argument *must* be a string, you can't use bytes.

The fix is the same as used with getattr() (where the second argument must
also always be a string); in the tokenizer, where we detect calls, if there
is something that looks like a call to open (and is not an attribute, so
the previous token is not a "." dot) then make sure that that second
argument is not converted to a `bytes` object instead.

There is some remaining issue where the current transformer will also rewrite

  open(f('foo')).

However this also affect function for which we perform similar rewrite
('getattr', 'setattr', 'hasattr', 'safehasattr') and will be dealt with in a
follow up.

  $ hg init
  $ touch a

  $ unset HGUSER
  $ echo "[ui]" >> .hg/hgrc
  $ echo "username= foo" >> .hg/hgrc
  $ echo "          bar1" >> .hg/hgrc

  $ hg ci -Am m
  adding a
  abort: username 'foo\nbar1' contains a newline
  
  [255]
  $ rm .hg/hgrc

  $ HGUSER=`(echo foo; echo bar2)` hg ci -Am m
  abort: username 'foo\nbar2' contains a newline
  
  [255]
  $ hg ci -Am m -u "`(echo foo; echo bar3)`"
  transaction abort!
  rollback completed
  abort: username 'foo\nbar3' contains a newline!
  [255]