I agree with Paul about the wording.  Note that the proposed platform-specific catch and raise are for crypt.py, not test_crypt.py.

If a test module should be skipped entirely, import_module is correct.  For more refinement, test_idle has, for example,

tk = import_module('tkinter')  # Also imports _tkinter.
if tk.TkVersion < 8.5:
    raise unittest.SkipTest("IDLE requires tk 8.5 or later.")

Testing only continues if tkinter and _tkinter and tk >= 8.5.

I presume that crypt = import_module('crypt') will only continue if crypt and  _crypt, which is what you want.  It is apparently not an error for _crypt to be missing on unix, just an inconvenience for people who expect it.

FYI, Individual test classes and methods can be skipped with
    @unittest.skipIf(condition, message)  # example
    @unittest.skipIf(sys.platform != 'darwin', 'test macOS-only code')
See the unittest doc for more, though apparently not needed here.