Mill service

[Danielhiversen]

Description:

Add a service to control the room temperature for the Mill heater

Pull request in home-assistant.io with documentation (if applicable): home-assistant/home-assistant.io#7235

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][ex-requir]).
• New dependencies are only imported inside functions that use them ([example][ex-import]).
• New or updated dependencies have been added to requirements_all.txt by running script/gen_requirements_all.py.
• New files were added to .coveragerc.

[houndci-bot]

Some files could not be reviewed due to errors:

Traceback (most recent call last):

Traceback (most recent call last):
  File "/home/linters/.local/bin/flake8", line 7, in 
    from flake8.main.cli import main
ModuleNotFoundError: No module named 'flake8'

[fabaff]

Looks good to me 🐦