Category-encoders

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* Added base class for contrast coding schemes in order to make them more maintainable
* Added hierarchical column feature in target encoder
* Fixed maximum recursion depth bug in hashing encoder

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* Introduce base class for encoders
* Introduce tagging system on encoders and use it to parametrize tests
* Drop support for python 3.5 and python 3.6
* Require pandas >=1.0
* Introduce f-strings
* Make BinaryEncoder a BaseNEncoder for base=2
* FutureWarning for TargetEncoder's default parameters
* Made all encoders re-fittable on different datasets (c.f. issue 122)
* Introduced tox.ini file for easier version testing

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* Fixed a bug with categorical data type in LeaveOneOut encoder
* Do not install examples as a package on its own

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* improved documentation
* fix bug in CatBoost encoder
* fix future warnings with pandas
* added tests for python 3.9 and 3.10 in pipeline
* fix treating np.NaN and python None equal
* only build docs on release
* unified conversion of inputs pandas objects that are used internally including some bugfixes.
* added quantile encoder and summary encoder

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* many bugfixes
* added count encoder

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* Added generalized linear mixed model encoder
* Added cross-validation wrapper
* Added multi-class wrapper
* Support for pandas >= 1.0.1
* Moved CI to github actions