Add modbus write coils

[janiversen]

Breaking change

Proposed change

the function write_coils was missing in ModbusHub. This was detected while writing the pymodbus test cases (soon
to come PR).

ModbusHub setup() contained an empty assert, which are dead code, since voluptuous secures that invalid config values are caught before entering the integration code. Dead code is removed.

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:

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.

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To help with the load of incoming pull requests:

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[probot-home-assistant]

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

[janiversen]

@MartinHjelmare I owe you a beer for your patience...the project should enable the possibility to donate beer etc. until the project does it is quite hard to pay my debt.

Problem solved, beer are on its way, thanks again for your patience....I learn a lot and use that when looking at other PRs

[MartinHjelmare]

Looks good!