Add Mill config flow

[Danielhiversen]

Breaking change

Mill can no longer be configured via configuration.yaml, configuration is now done though UI

Proposed change

Add config flow support to Mill integration

Type of change

• Dependency upgrade
• Bugfix (non-breaking change which fixes an issue)
• New integration (thank you!)
• New feature (which adds functionality to an existing integration)
• Breaking change (fix/feature causing existing functionality to break)
• Code quality improvements to existing code or addition of tests

Example entry for configuration.yaml:

# Example configuration.yaml

Additional information

• This PR fixes or closes issue: fixes #
• This PR is related to issue:
• Link to documentation pull request: Mill config flow home-assistant.io#13273

Checklist

• The code change is tested and works locally.
• Local tests pass. Your PR cannot be merged unless tests pass
• There is no commented out code in this PR.
• I have followed the development checklist
• The code has been formatted using Black (black --fast homeassistant tests)
• Tests have been added to verify that the new code works.

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

• Documentation added/updated for www.home-assistant.io

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

• The manifest file has all fields filled out correctly.
  Updated and included derived files by running: python3 -m script.hassfest.
• New or updated dependencies have been added to requirements_all.txt.
  Updated by running python3 -m script.gen_requirements_all.
• Untested files have been added to .coveragerc.

The integration reached or maintains the following Integration Quality Scale:

• No score or internal
• 🥈 Silver
• 🥇 Gold
• 🏆 Platinum

[probot-home-assistant]

Hey there @Danielhiversen, mind taking a look at this pull request as its been labeled with a integration (mill) you are listed as a codeowner for? Thanks!
_{^{(message by CodeOwnersMention)}}

[balloob]

(comment not related to line but to file)

For a future PR, we should register the service using entity_platform.async_register_entity_service move the service registration to __init__.py and make sure it works across config entries.

[bdraco]

Looks like just the missing coverage for line 43,44 in config_flow.py and this is ready to go.

# pytest --cov=homeassistant/components/mill/ --cov-report term-missing -- tests/components/mill/
Test session starts (platform: linux, Python 3.8.1, pytest 5.3.5, pytest-sugar 0.9.2)
rootdir: /root/home-assistant, inifile: setup.cfg
plugins: requests-mock-1.7.0, aiohttp-0.3.0, cov-2.8.1, sugar-0.9.2, timeout-1.3.3, profiling-1.7.0, parallel-0.1.0
collecting ... 
 tests/components/mill/test_config_flow.py ✓✓✓                                                                                                                                                                                                                   100% ██████████

----------- coverage: platform linux, python 3.8.1-final-0 -----------
Name                                           Stmts   Miss  Cover   Missing
----------------------------------------------------------------------------
homeassistant/components/mill/config_flow.py      26      2    92%   43-44


Results (0.65s):
       3 passed
root@ha-dev:~/home-assistant# vim homeassistant/components/mill/config_flow.py  

[bdraco]

Nice job @Danielhiversen