Replaced .iteritems() with six.iteritems() for Python 2 and 3 compat

[igormp]

Fixed some python 2 leftovers, as discussed in #267.
Also formatted code according to Black, I hope that's not a problem.

This possibly breaks some python 2 compatibility.

[igormp]

As an addendum, I've noticed the usage of six.iteritems() in order to keep both python 2 and 3 compatibility. I wasn't sure if I should've kept using it instead of .items(), but I can't see why we should support Python 2 anyway.

[tataganesh]

@igormp Thank you for the contribution. But isn't .items() supported in both Python2 and Python 3?

[igormp]

It is indeed, but might be way slower in Python 2 (at least according to this).

[pietermarsman]

but I can't see why we should support Python 2 anyway.

For now we support both python 2 and 3. Perhaps that will be changed in the future. Dropping python 2 is discussed (as you already know) in #194.

Also formatted code according to Black, I hope that's not a problem.

Only format the lines you actually change. Formatting lines written by others messes up the git line history which can come in handy if you want to figure out why things are made in a particular way.

As an addendum, I've noticed the usage of six.iteritems() in order to keep both python 2 and 3 compatibility.

As I see it there are 3 options: .iteritems(), .items(), and six.iteritems(). The does not exist in python 3, so we should not use that. There are still 8 occurances of .iteritems() and they should be replaced in this PR. .items() only works well in python 3, in python 2 it can cause memory issues because it returns a list instead of a generator, so we should not use it. I think six.iteritems() is the best solution because it works in both 2 and 3 and does not have memory issues.

In short: I suggest we replace all occurances of .iteritems() and .items() with six.iteritems().

[pietermarsman]

This is an (active) duplicate of #123.

[igormp]

Ok, redid it. I can squash the commits after someone reviews the new changes.

I also think that it's a good idea to rename the PR since we're going to keep six.

[pietermarsman]

This PR is looking good, it is a clear improvement and makes our code more consistent. I do not have any comments.

[igormp]

Squashed the commits and changed the title, everything is good to go.