Pyreadr

* fixed signature 84

* fixed numpy.float deprecation
* fixed truncation of files when writing on windows

* Introducing new pandas data types Float64Dtype and Float32Dtype when writing.
This requires pandas > 1.2.0. Python 3.6 taken out of wheels as there
is no 1.2.0 version.

* fixed bug when translating datetime to string when writing.

* Added support for pathlib, solves 70

* removed warnings about np.bool and np.object