Change PosixPath.readlink() to PosixPath.resolve() for compatibility with Python <3.9

The .readlink() function is not available in the pathlib library prior to Python 3.9. Replacing it with a call to .resolve() for the same result.

Fixes:
Closes: #481
Related:

Checklist:

• Adhere to the development process
• Conform to the style guide
• Verify that the changes work as expected on real hardware
• Add automated tests cases
• Verify that all (other) automated tests (still) pass
• Update the README and other applicable documentation pages
• Update the liquidctl.8 Linux/Unix/Mac OS man page
• Update or add applicable docs/*guide.md device guides
• Submit relevant data, scripts or dissectors to https://github.com/liquidctl/collected-device-data

New CLI flag?

• Adjust the completion scripts in extra/completions/

New device?

• Regenerate extra/linux/71-liquidctl.rules (instructions in the file header)
• Add entry to the README's supported device list with applicable notes (at least en)

New driver?

• Document the protocol in docs/developer/protocol/