As we no longer support Python 2, there is no reason to
keep this dependency. This commit removes all usages
of six and removes it from dependencies.

There are some stale mentions in docs / comments about
Python versions that are no longer supported. There
are also some workarounds to make driver work with those
versions. This commit removes all mentions and workarounds
that I was able to find.

six.iterkeys() returns an iterator, but Python's
dict.keys() does not, so to pass it to iter() it needs
to be first passed trough iter().