Use consts and string formatting

[fabaff]

Description:

Use consts and string formatting.

Part of preparation for #7108.

Example entry for configuration.yaml (if applicable):

notify:
  - platform: instapush
    name: instapush
    api_key: !secret instapush_api
    app_secret: !secret instalpush_secret
    event: msg
    tracker: state

Checklist:

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

• Local tests with tox run successfully. Your PR cannot be merged unless tests pass

[mention-bot]

@fabaff, thanks for your PR! By analyzing the history of the files in this pull request, we identified @lwis, @balloob and @pvizeli to be potential reviewers.