Add opentherm_gw binary sensor support

[mvn23]

Description:

Add binary sensor support to the OpenTherm Gateway component

Related issue (if applicable): ref. #16670

Pull request in home-assistant.io with documentation (if applicable): home-assistant/home-assistant.io#6975

Example entry for configuration.yaml (if applicable):

opentherm_gw:
  device: /dev/ttyUSB0
  monitored_variables:
  - flame_on
  - slave_ch_active
  - slave_service_required
  - slave_water_overtemp
  - otgw_gpio_a_state

Checklist:

• The code change is tested and works locally.
• Local tests pass with tox. Your PR cannot be merged unless tests pass

If user exposed functionality or configuration variables are added/changed:

• Documentation added/updated in home-assistant.io

If the code communicates with devices, web services, or third-party tools:

• New dependencies have been added to the REQUIREMENTS variable (example).
• New dependencies are only imported inside functions that use them (example).
• New or updated dependencies have been added to requirements_all.txt by running script/gen_requirements_all.py.
• New files were added to .coveragerc.

[MartinHjelmare]

Shouldn't the binary_sensor entity be non polling?

[mvn23]

Should be non-polling indeed. Thanks for noticing!

[MartinHjelmare]

Are the other platforms implemented as non polling?

[mvn23]

After your comment I checked the climate and sensor platforms as well to make sure, they are both non-polling. Don't know how I missed it here...

[MartinHjelmare]

Good! Can be merged when build passes.

[mvn23]

Awesome! I was already hoping to get this through for 0.81. Thanks for reviewing!