Remove Python 2 support, six dependency

[rdb]

Thanks for this great project!

Python 2 is ancient history by now, and as pointed out in #618, pyjnius does not even work with Python versions under 3.6 at this point.

This PR removes the compatibility hacks for Python 2 and drops the now unnecessary dependency on the six package.

[misl6]

We're in the process of releasing version 1.4.2, which will be a bugfix one. As this one will break Python 2 compatibility, I've added it to milestone 1.5.0.

@rdb We now have some conflicts, can you take care of it?

[rdb]

Sorry for the delay, I've addressed the merge conflicts and rebased. Tests pass locally on Linux.

As this one will break Python 2 compatibility, I've added it to milestone 1.5.0.

This totally makes sense and is no problem for me, just wanted to point out that no Python 2 compatibility is broken by this PR, since pyjnius is already incompatible with Python 2 due to the use of f-strings :)