Remove unittest code from non-test code

I see these imports of unittest in

GitPython/git/objects/submodule/base.py

Line 6 in a8591a0

from unittest import SkipTest
GitPython/git/util.py

Line 17 in f3265bd

from unittest import SkipTest

I generally think that test code should not be built into the business logic. Is there a reason why using a skipIf decorator is insufficient?

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