py-MechanicalSoup: update to 0.8.0

Version 0.8

Main changes:
* `Browser` and `StatefulBrowser` can now be configured to raise a
  `LinkNotFound` exception when encountering a 404 Not Found error.
  This is activated by passing `raise_on_404=True` to the constructor.
  It is disabled by default for backward compatibility, but is highly
  recommanded.
* `Browser` now has a `__del__` method that closes the current session
  when the object is deleted.
* A `Link` object can now be passed to `follow_link`.
* The user agent can now be customized. The default includes
  `MechanicalSoup` and its version.
* There is now a direct interface to the cookiejar in `*Browser`
  classes (`(set|get)_cookiejar` methods).
* This is the last MechanicalSoup version supporting Python 2.6 and
  3.3.

Bug fixes:
* We used to crash on forms without action="..." fields.
* The `choose_submit` method has been fixed, and the `btnName`
  argument of `StatefulBrowser.submit_selected` is now a shortcut for
  using `choose_submit`.
* Arguments to `open_relative` were not properly forwarded.

Internal changes:
* The testsuite has been greatly improved. It now uses the pytest API
  (not only the `pytest` launcher) for more concise code.
* The coverage of the testsuite is now measured with codecov.io. The
  results can be viewed on:
  https://codecov.io/gh/hickford/MechanicalSoup
* We now have a requires.io badge to help us tracking issues with
  dependencies. The report can be viewed on:
  https://requires.io/github/hickford/MechanicalSoup/requirements/
* The version number now appears in a single place in the source code.


(adam)