Pyndl

This release 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
- fixed a bug in the writing of file names in the `corpus` module
- 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`
- we stop signing the package with a gpg key with this release as it is not supported by `pypoetry` and seems not to have been adapted by the Python community
- adds benchmark to the documentation, which compares pyndl with ndl2 and ndl
- restructures the documentation to improve readability

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

This fixes the authorship of Elnaz at several locations. To trigger Zenodo and readthedocs we publish this release.

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
- fixed a bug in the writing of file names in the `corpus` module
- 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`
- we stop signing the package with a gpg key with this release as it is not supported by `pypoetry` and seems not to have been adapted by the Python community

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.

* improves the documentation on Widrow-Hoff learning

* adds Widrow-Hof learning on Linux (real_to_real, real_to_binary, binary_to_real)
* adds fast correlation code
* drop python 3.6 support and add python 3.9 support
* changes API from `number_of_threads / number_of_processes` to `n_jobs`
* exposes make_data_array as ndl.data_array(...) and adds tests
* adds learning events from generators in ndl.ndl
* changes tuples into namedtuples for return values
* adds sanity check for too many cues or outcomes
* checks if outcomes_vectors, cue_vectors and weights are c_contiguous before passing it to cython functions
* adds scipy, packaging as dependencies

Co-authored-by: Elnaz Shafaei <elnaz.shafaeigmail.com>
and Marc Weitz <marc.weitzonlinehome.de>

- changes tests to be run on mac with threading
- drops support for python 3.5 due to the usage of format strings
- adds testing for python 3.8
- corrects grand information in zenodo and adds Acknowledgements in readme