Add StrEnum for device_tracker SourceType

[cdce8p]

Deprecation

Custom integrations only: Deprecate device_tracker SOURCE_TYPE_* constants. Use the SourceType enum instead.

Proposed change

Add new StrEnum for device_tracker SOURCE_TYPE_* constants. This PR only updates the device_tracker integration itself and the device_tracker platform for mobile_app as example. The remaining integration platforms will be updated in follow up PRs if this one is accepted.

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)
• Deprecation (breaking change to happen in the future)
• Breaking change (fix/feature causing existing functionality to break)
• Code quality improvements to existing code or addition of tests

Additional information

• This PR fixes or closes issue: fixes #
• This PR is related to issue:
• Link to documentation pull request: Update device_tracker source_type properties developers.home-assistant#1414

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.
• For the updated dependencies - a link to the changelog, or at minimum a diff between library versions is added to the PR description.
• Untested files have been added to .coveragerc.

The integration reached or maintains the following Integration Quality Scale:

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

To help with the load of incoming pull requests:

• I have reviewed two other open pull requests in this repository.

[probot-home-assistant]

Hey there @home-assistant/core, mind taking a look at this pull request as it has been labeled with an integration (mobile_app) you are listed as a code owner for? Thanks!
_{^{(message by CodeOwnersMention)}}

[probot-home-assistant]

Hey there @home-assistant/core, mind taking a look at this pull request as it has been labeled with an integration (device_tracker) you are listed as a code owner for? Thanks!
_{^{(message by CodeOwnersMention)}}

[epenet]

Should this have a corresponding dev blog entry?
eg. home-assistant/developers.home-assistant#1295

[cdce8p]

Should this have a corresponding dev blog entry?
eg. home-assistant/developers.home-assistant#1295

Opened home-assistant/developers.home-assistant#1414
As draft until the PR is accepted.

[MartinHjelmare]

Thanks!