Timezonefinder

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* added option to specify the location of the binary data files to use. making it possible to easily point to own compiled data. also load timezone names json from this location
* make timezone names a class attribute (instead of a global variable)
* simplify code for opening and closing multiple binary files
* added tests for a specified path to the data
* testing multiple python3 versions automatically
* pinned new requirements
* importlib_resources removed from the dependencies
* added a documentation at: https://timezonefinder.readthedocs.io/en/latest/
* added contribution guidelines

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* updated the data to `2019b <https://github.com/evansiroky/timezone-boundary-builder/releases/tag/2019b>`__
* added description of using vectorized input in readme

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* clarification of readme: referenced latest `timezonefinderL` release, better rst headlines, updated shield.io banner syntax
* clarification of speedup times (exponential notation)
* removed `six` and py2 dependency from tests
* minor updates to publishing routine
* minor improvement in timezone_at(): conversion coordinates to int later only when required

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* updated the data to `2019a <https://github.com/evansiroky/timezone-boundary-builder/releases/tag/2019a>`__

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* BUGFIX: fixing 77 (missing dependency in setup.py)

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* ATTENTION: Dropped Python2 support (72)! `six` dependency no longer required.
* BUGFIX: fixing 74 (broken py3 with numba support)
* added `in_memory`-mode (adapted unit tests to test both modes, added speed tests and explanation to readme)
* use of timeit in speed tests for more accurate results
* dropped use of kwargs_only decorator (can be implemented directly with python3)