Fix regex in salt.modules.apt._get_upgradable

Third capture group assumes no spaces in the source name, and certain
PPAs (Salt's own being one of them) contain a space. Modified the regex
so that it should work with any apt source. This fixes #3733. Also made
a couple pep8 fixes.
Merge pull request #3879 from archtaku/issue3733

Fix regex in salt.modules.apt._get_upgradable