Add OpenTherm Gateway climate platform

[mvn23]

Description:

Add support for the OpenTherm Gateway.

Pull request in home-assistant.github.io with documentation (if applicable): home-assistant/home-assistant.io#6118

Example entry for configuration.yaml (if applicable):

climate:
  - platform: opentherm_gw
    device: /dev/ttyUSB0

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.github.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.

[mvn23]

Thank you for the very thorough review. Looks like there is still quite some work to do, but that was expected since it's my first real contribution to Home Assistant. I will address all concerns over the next few days.

[MartinHjelmare]

Shouldn't polling be set to False for this entity?

[mvn23]

Shouldn't polling be set to False for this entity?

I've been looking for a way to do this, but could not find it in the dev docs before. Seems I missed the "Entities Introduction" part and relied too much on examples from other platforms.
I will have a look and prepare another commit tonight or tomorrow.
Thanks for the feedback!

[MartinHjelmare]

Nice! Two small comments.

[MartinHjelmare]

Great!