Pyndl: Naive discriminative learning in python

This is a beta release that includes the following changes:

• for preprocess.create_event_file we rename symbols to allowed_symbols to make this argument more self-explainable
• the default value of allowed_symbols is now most inclusive and it is a keyword only argument
• allowed_symbols can take a filter function, which defines on a character by character basis, if this character should be included into the event file
• no deprecation warnings will be introduced in this release and this release breaks some API, so please check, if your code still runs
• wheels for linux should be released from now on
• test coverage is increased and explicitly tests filter functionality and a non-ascii charater sets; running --runslow is successfully completes all tests now
• package maintenance is now done with pypoetry instead of a combination of setuptools, tox, and a Makefile

Thanks to @Trybnetic and @dekuenstle , who made this release possible and to @frankier who raised good issues and pointed out areas of improvement.

Missing is an overhaul of the documentation, which will be addressed in September.