Remove unittest code from non-test code

[mwatts15]

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?

[Byron]

Good catch, I absolutely agree! Even though I am pretty sure there was a very good reason to do that at some point in time. With the current state of windows test, for which these dependencies seemingly have been introduced. Maybe @ankostis can provide more context, who did magnificent work a while ago to get windows back into shape.