_pythonSymlinkCheck does not work on cosmic+

[Impact]
_pythonSymlinkCheck() checks that python links to the default python version. In cosmic, python links to python2 however, so we need to check if that's the case, and then check what python2 links to.

This breaks upgrades from cosmic to later versions, and partial upgrades on cosmic (that is, running _pythonSymlinkCheck).

[Test case]
Run /usr/lib/ubuntu-release-upgrader/do-partial-upgrade make sure it does not error out in the python symlink check, for example, by getting to the UI where it shows the upgrades.

[Regression potential]
Practically none for cosmic users, as the function currently is guaranteed to fail.

When running the code on older releases (which happens due to the way release upgrading works), the code now allows python to symlink to python2, even where that is not the case in the packaging. But anything else would complicate the check a bit too much.