test: support Python 3.10 metadata parsing

[lukisch]

Restores the declared Python 3.10 CI lane by using stdlib omllib when available and the smallest compatible omli fallback otherwise.

Scope:

• test-only import compatibility
• conditional dev dependency for Python <3.11

Validation:

• pytest: 55 passed on local Python 3.12
• git diff --check
• Python 3.10 is not installed locally; GitHub CI is the required runtime proof.