Py-buzz

* Added `exc_builder` option to all methods.

* Added `enforce_defined()` to Buzz class.

* Added `enforce_defined()` function with tests and docs.

* Added `raise from` in handle_errors (how did I miss this for so long?)

* Returned support for Python 3.6

* Moved functions to standalone tools module
* Made Buzz re-use these methods
* Changed `handle_errors` `exception_class` parameter to `handle_exc_class`
* Changed `handle_errors` to send `DoExceptParams` as a single parameter
* Removed `sanitize_err_string` as it should not be needed after 2.0.0
* Removed `re_raise` parameter from handle_errors in tools.
* Made passing `None` to raise_exc_class indicate no new excepton should be raised
* Added a Makefile for running quality checks easier.
* Updated and added examples.
* Updated docs a lot.
* Dropped support for Python 3.6