Fix flake8 E721 complaints after Python upgrade

[mikkonie]

These should be fixed. I believe they are the result of Python version upgrades. I encountered them while doing #1922.

For now I'll just add E721 to ignore. Note: Remove after fixing these :)

./samplesheets/io.py:284:12: E721 do not compare types, for exact checks use `is` / `is not`, for instance checks use `isinstance()`
./samplesheets/io.py:332:12: E721 do not compare types, for exact checks use `is` / `is not`, for instance checks use `isinstance()`
./samplesheets/io.py:410:16: E721 do not compare types, for exact checks use `is` / `is not`, for instance checks use `isinstance()`
./samplesheets/io.py:465:39: E721 do not compare types, for exact checks use `is` / `is not`, for instance checks use `isinstance()`
./samplesheets/models.py:122:14: E721 do not compare types, for exact checks use `is` / `is not`, for instance checks use `isinstance()`
./samplesheets/models.py:127:12: E721 do not compare types, for exact checks use `is` / `is not`, for instance checks use `isinstance()`
./samplesheets/models.py:129:14: E721 do not compare types, for exact checks use `is` / `is not`, for instance checks use `isinstance()`
./samplesheets/models.py:131:14: E721 do not compare types, for exact checks use `is` / `is not`, for instance checks use `isinstance()`
./samplesheets/utils.py:143:8: E721 do not compare types, for exact checks use `is` / `is not`, for instance checks use `isinstance()`

[mikkonie]

Done.