Minor cleanups of the code to be more PEP8-conform ( https://www.python.org/dev/peps/pep-0008/ )

Kai wrote at meld-list@gnome.org to this topic: 

"
The problem with PEP8-ing all of the current code is that it adds
noise to our git history. The problem isn't the commits themselves,
but rather the git annotate (i.e., blame) output, which will show the
spurious not-really-a-change commit introduced by PEP8 for any
affected code. This can be particularly annoying in older, large files
(i.e., don't bother trying to fix filediff or dirdiff).

As a general rule, I'll probably accept patches for PEP8ing files
where the changes aren't too big, particularly when the file is
almost-PEP8 already. For some files where the 79 character limit has
already been tossed out the window, I wouldn't bother trying to
reinstate that one just yet.

It's worth mentioning two PEP8 caveats. Firstly, PEP8 allows backslash
continuations for long lines, but we don't use them (by which I
mean... we don't use them *any more*). Secondly, PEP8 allows aligned
hanging indents for continued lines, but I would strongly discourage
this.
"