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This patch avoids importing test runners such as :pypi:`pytest`, :pypi:`unittest2`,
or :pypi:`nose` solely to access their special "skip test" exception types -
if the module is not in :obj:`sys.modules`, the exception can't be raised anyway.
This fixes a problem where importing an otherwise unused module could cause
spurious errors due to import-time side effects (and possibly ``-Werror``).