webtest-*.ebuild: Install lint3 for Python 3

webtest-*.ebuild: Install lint3 for Python 3

As I reported in Gentoo's #524724 [1]:

  $ docker run --rm -i -t wking/gentoo-portage:20140911 /bin/bash
  # emerge -av dev-python/webtest
  # python3.3 -c 'import webtest'
  Traceback (most recent call last):
    File "<string>", line 1, in <module>
    File "/usr/lib64/python3.3/site-packages/webtest/__init__.py", line 11, in <module>
      from webtest.app import TestApp
    File "/usr/lib64/python3.3/site-packages/webtest/app.py", line 41, in <module>
      from webtest import lint3 as lint
  ImportError: cannot import name lint3

I've also trimmed the Manifest, because my overlay uses thin, unsigned
manifests.  Feel free to re-thicken if you prefer.

I've also skipped the ChangeLog entry, because folks can just look at
this commit message.  Feel free to extract a ChangeLog entry from this
if you prefer.

[1]: https://bugs.gentoo.org/show_bug.cgi?id=524724