Add multi select support to config validation and to custom serializer

[Kane610]

Breaking change

Proposed change

This adds backend support to previously opened PR to frontend by Bram.

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: Add multi select component to ha-form frontend#4247
• 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
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• 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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[codecov]

Codecov Report

Merging #31798 into dev will increase coverage by 0.00%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##              dev   #31798   +/-   ##
=======================================
  Coverage   94.67%   94.68%           
=======================================
  Files         763      763           
  Lines       55126    55148   +22     
=======================================
+ Hits        52192    52215   +23     
+ Misses       2934     2933    -1     

Impacted Files	Coverage Δ
homeassistant/components/simplisafe/const.py	100.00% <0.00%> (ø)	⬆️
homeassistant/components/template/cover.py	97.71% <0.00%> (+1.36%)	⬆️

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[bramkragten]

Might be doing something wrong, but I get errors trying this:

    data["data_schema"] = voluptuous_serialize.convert(schema)
  File "/Users/bram/Documents/projects/home-assistant/home-assistant/venv/lib/python3.7/site-packages/voluptuous_serialize/__init__.py", line 38, in convert
    pval = convert(value, custom_serializer=custom_serializer)
  File "/Users/bram/Documents/projects/home-assistant/home-assistant/venv/lib/python3.7/site-packages/voluptuous_serialize/__init__.py", line 108, in convert
    raise ValueError('Unable to convert schema: {}'.format(schema))
ValueError: Unable to convert schema: <function multi_select.<locals>.validator at 0x11682b4d0>

With:

            data_schema=vol.Schema(
                {
                    vol.Optional("test"): cv.multi_select({"paulus": "Paulus", "robban": "Robban"}),
                }

[balloob]

Oh right 🤦‍♂ multi_select returns a local function that we try to compare to multi_select, which is obviously not true

We should probably make MultiSelect a class with a __call__ so that we can do isinstance.

class multi_select:
    def __init__(options)
    def __call__(value)

[Kane610]

@balloob @bramkragten something like this? #31828