stbt.py: [pylint 1.0] use new-style classes throughout

Pylint 1.0 insists on new-style classes, meaning that custom classes
must inherit from either `object` or another new-style class.
pylint.conf: [pylint 1.0] Ignore unused variables beginning with "_"

In pylint 1.0 the default value for "dummy-variables-rgx" changed from
"_" to "_$". So unused parameters like "_bus" in stbt.py's
Display.on_error were triggering the W0613 checker (Unused argument).
stbt lint: [pylint 1.0] work with either logilab.astng or astroid

Logilab renamed their `astng` module to `astroid`, but almost all of the
classes and functions that it includes have remained with the same name,
so we just include that import line in the 1.0 < pylint <= 1.0 try/except
block.

Note that I moved the E0611 (no-name-in-module) pylint warning to wrap
the entire block, and added F0401 (import-error) for `logilab.astng`.
stbt lint: '.png' does not count as a missing image file

The image path verifier should not raise an error for finding a string
that is *just* '.png' by declaring that the image does not exist on file.
It is completely understandable that users might want to use '.png' in
their code with it *not* referring to an image file. It would have to be
a hidden file, and I don't think anyone is crazy enough to use a hidden
file in their test scripts. '.png' is used as the `suffix` parameter
to a call to mktmp in stbt.py.

Note that the original regex was flawed in searching for r'.png' as this
matches any 4 character string ending with 'png' with '.' acting as a
wildcard. The new regex only returns True if it finds at least one
character followed by '.png', thus exluding simple '.png'
stbt lint: Don't refuse to work with pylint 1.x

The problems described in 97eada33 were fixed in our earlier commit
b893f23b.

The pylint problem described in 65a38380 has been fixed in pylint 1.0.1.
Fixes to pass pylint 1.1.0

The errors in irnetbox-proxy were W0633 (attempting to unpack a
non-sequence) because `IRNetBoxProxy.get_message_from_irnet` returns
`None` when its parameter `expect_sync_response` is False; pylint
doesn't figure out that these calls are guaranteed to return a sequence
because `expect_sync_response` is True. C0301 is "Line too long", and
is necessary for older versions of pylint that require it even when the
line length was exceeded by a pylint comment.

The remaining errors were C0325 (Unnecessary parens after 'if',
'return', or 'print' keyword). In the case of `print`, these same files
already had instances without parentheses, so we're not hurting python 3
compatibility.

I can't disable new checkers altogether in pylint.conf, because older
versions of pylint (0.28) will raise a `pylint.utils.UnknownMessage`
exception. At least I can disable new checkers with `# pylint: disable`
comments in the code itself, as long as I disable E0012 (Bad option
value; Used when a bad value for an inline option is encountered) in
pylint.conf.

Tested with pylint 0.28.0, 1.0.0, and 1.1.0 from macports.
Merge branch 'pylint-1.0-compatibility'

* pylint-1.0-compatibility:
  Fixes to pass pylint 1.1.0
  stbt lint: Don't refuse to work with pylint 1.x
  stbt lint: '.png' does not count as a missing image file
  stbt lint: [pylint 1.0] work with either logilab.astng or astroid
  pylint.conf: [pylint 1.0] Ignore unused variables beginning with "_"
  stbt.py: [pylint 1.0] use new-style classes throughout