Don't import `unittest` in debugging.py

[bluetech]

In the pytest code we try very hard to avoid importing unittest if it's not already imported, even in the unittest plugin. But we slipped here:

pytest/src/_pytest/debugging.py

Line 14 in e826510

import unittest

We should handle it similarly to other places by checking if unittest is in sys.modules first.

[bluetech]

While looking at this code in debugging.py, I figure we really shouldn't treat unittest.SkipTest as an interactive exception in the first place, i.e. it shouldn't trigger pytest_exception_interact at all, similarly to how pytest's own Skipped doesn't trigger it. Then the skipping concerns are isolated in runner.py and debugging.py doesn't need to care about unittest.