Fix minor Ruff/flake8 warnings
    
    ```
    pungi/checks.py:575:17: F601 [*] Dictionary key literal `"type"` repeated
    pungi/phases/pkgset/pkgsets.py:617:12: E721 Do not compare types, use `isinstance()`
    tests/test_pkgset_source_koji.py:241:16: E721 Do not compare types, use `isinstance()`
    tests/test_pkgset_source_koji.py:244:16: E721 Do not compare types, use `isinstance()`
    tests/test_pkgset_source_koji.py:370:16: E721 Do not compare types, use `isinstance()`
    tests/test_pkgset_source_koji.py:374:20: E721 Do not compare types, use `isinstance()`
    ```
    
    Signed-off-by: Timothée Ravier <tim@siosm.fr>