Switch to enum.StrEnum

[jamesonuk]

Motivation:

Just avoiding future deprecation and logging warning of old alias as per https://developers.home-assistant.io/blog/2024/04/08/deprecated-backports-and-typing-aliases/

Changes:

import enum.StrEnum rather than homeassistant.backports.enum.StrEnum

Related issue (check one):

• [x] fixes #1351
• [ ] there is no related issue ticket

Checklist (check one):

• [ ] I did not change any code (e.g. documentation changes)
• [x] The code change is tested and works locally.

Test-Hardware list (for code changes)

HA Version: 2024.5.3 Zigbee2MQTT Version: 1.37.1 TRV Hardware: Sonoff TRVs 1.1.5

New device mappings

• [x] I avoided any changes to other device mappings
• [x] There are no changes in climate.py