Add a warning that honeywell/EU is to be deprecated

[zxdavb]

Description:

The honeywell component actually contains two mutually-exclusive integrations, based upon either the somecomfort (US) or evohome-client (EU) client libraries. The evohome functionality is better provided in the evohome component, which uses the same client library.

This PR will add a warning to users of the honeywell component that support for EU-based systems is deprecated in favour of the evohome component.

This warning does not apply to / will not be seen by users of US-based systems.

Related issue (if applicable): None

Pull request in home-assistant.io with documentation (if applicable): None

Checklist:

• The code change is tested and works locally.
• Local tests pass with tox. Your PR cannot be merged unless tests pass
• There is no commented out code in this PR.

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:

• The manifest file has all fields filled out correctly (example).
• New dependencies have been added to requirements in the manifest (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.

If the code does not interact with devices: N/A

• Tests have been added to verify that the new code works.

[amelchio]

Probably adding a specific (not too distant) version number could speed up migrations.