Add support of context managers in :mod:`unittest`: methods
:meth:`~unittest.TestCase.enterContext` and
:meth:`~unittest.TestCase.enterClassContext` of class
:class:`~unittest.TestCase`, method
:meth:`~unittest.IsolatedAsyncioTestCase.enterAsyncContext` of class
:class:`~unittest.IsolatedAsyncioTestCase` and function
:func:`unittest.enterModuleContext`.
