Update to 3.0.

The new major version number doesn't indicate any deliberate API incompatibility.
We have endeavoured to avoid breaking existing APIs. However, pexpect is under
new maintenance after a long dormancy, so some caution is warranted.

* A new :ref:`unicode API <unicode>` was introduced.
* Python 3 is now supported, using a single codebase.
* Pexpect now requires at least Python 2.6 or 3.2.
* The modules other than pexpect, such as :mod:`pexpect.fdpexpect` and
  :mod:`pexpect.pxssh`, were moved into the pexpect package. For now, wrapper
  modules are installed to the old locations for backwards compatibility (e.g.
  ``import pxssh`` will still work), but these will be removed at some point in
  the future.
* Ignoring ``SIGHUP`` is now optional - thanks to Kimmo Parviainen-Jalanko for
  the patch.

(wiz)