Py-entitymatching

* Fixed various errors in setup.py when pip installing
* Adjusted setuptools.setup project name to match name on PyPI

* Dropped support for Python 3.6.
* Added support for Python 3.10 and 3.11.
* Switched from Nose to vanilla Unittest.
* Replaced Travis and Appveyor CI testing with Github Actions.

* Dropped support for Python 2 and 3.5.
* To support Python 3.8, updated the function
py_entitymatching.matcher.matcherutils.impute_table() to use current scikit-learn's
SimpleImputer; see issue 127.

* Started tracking release changes in CHANGES.txt.
* Minorly updated usage of Pandas to prevent a dependency on downgraded versions.
* Added stricter scikit-learn dependency requirements to preserve old Imputer API; will change in future release.
* Users can now provide metadata for blackbox features (ex. left_attribute, right_attribute, etc.).
* This is the last version of py_entitymatching that will support Python 2 and Python 3.5.