execnet-1.2.0-r1.ebuild: Update python* entries in PYTHON_COMPAT

execnet-1.2.0-r1.ebuild: Update python* entries in PYTHON_COMPAT

Python 2.6 is no longer in the tree, but 3.3 and 3.4 are.

I went with a revbump here (in contrast to bc861e9e, Add support for
the new PyPy slotting, 2014-03-31).  From the devmanual [1]:

  Ebuilds should have their -rX incremented whenever a change is made
  which will make a substantial difference to what gets installed by
  the package — by substantial, we generally mean "something for which
  many users would want to upgrade".

Whether or not the 2.6, 3.3, or 3.4 versions of the package are
installed for a given PYTHON_TARGETS seems like a substantial
difference to me ;).

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
is commit message.  Feel free to extract a ChangeLog entry from this
if you prefer.

[1]: http://devmanual.gentoo.org/general-concepts/ebuild-revisions/index.html