Added
* Use `poetry` to manage dependencies, build and package Lexicon.
* Add integration tests for Mac OS X
Changed
* The `beautifulsoup4` dependency has been integrated to the core
ones for generic purpose. As a consequence `henet`, `easyname` and `gratisdns`
providers do not have optional dependencies anymore.
* Update Docker image to use Python 3.8, and install Lexicon with full extras.
Removed
* Remove support for Python 2.7.
* Remove support for Python 3.5.
* Remove the extra `security` from `requests` dependency which
does not make sense anymore on recent versions of Python.