Gpiodmonitor

* completely rewritten codebase
* can now be used as a context manager
* now supports type-hinting (so it needs python >= 3.7

* completely rewrite code
* allow to monitor for arbitrarily long button presses

Iniitial release.

[Download from PiPy](https://pypi.org/project/gpiodmonitor/) using `pip install gpiodmonitor`.