* add CONF_STATE_CLASS to const.py
* move to `sensor/__init__.py`
* move to sensor/const.py
* Revert "move to sensor/const.py"
This reverts commit 604d0d066b.
* move it to `sensor/const.py` but import it from `sensor/__init__.py`
* add Modbus
* Added availability_template to Template Sensor platform
* Added to test for invalid values in availability_template
* Updated AVAILABILITY_TEMPLATE Rendering error
* Moved const to package Const.py
* Fix import order (pylint)
* Moved availability_template rendering to common loop
* Removed 'Magic' string
* Cleaned up const and compare lowercase result to 'true'
* Device is unavailbale if value template render fails
* Converted test ot Async and fixed states
* Comverted back to using boolean for _availability
* Fixed state check in test
