Remove unittest code from non-test code #790

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?