Py2&3 fixes

[tomppa65]

This branch includes changes that should enable zeeplibrary to work with both python 2.7 and python 3.
There were not many actual changes required to make the library work with python 3. The pull request branch does, however, contain many commits and some additional changes. I've

• tried to make some linters reasonably happy (correcting indentation and import order and such),
• added a set of unit tests for ZeepLibrary's keyword methods (except for call_operation) to be run with pytest
• added tox.ini file to enable running the unit tests with both py27 and py39 using tox (must install it separately first)
• added requirements.txt file to list all python libraries needed to run the unit tests. tox uses that to build a virtual env for each python version.