Fix regex in _get_upgradable

We're not using the data after the version number in the apt-get output,
so don't bother trying to write an elaborate regex for parts of the
string about which we don't care. Fixes #4107.

Also made some pep8 fixes to make pylint happy.
Merge pull request #4128 from terminalmage/issue4107

Fix regex in _get_upgradable