`test/test_youtube_signature.py` not excluded in target `offlinetest`

[ghost]

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The tests in test/test_youtube_signature.py need internet access (they use compat_urlretrieve()).
However, this file is not excluded in the offlinetest target in Makefile.

Suggested fix: add --exclude test_youtube_signature.py to this target.

[yan12125]

Thanks for the report - fixed.