Clean up asyncio tests

We finish the cleanup of `test_asyncio.py` done in c60dd14
and in 204647b by removing remnants of support of Pythons with no
`asyncio` module because all Python interpreters supported by freezegun
already support `asyncio`.

tests/test_asyncio.py

@@ -1,15 +1,10 @@
 import asyncio
 import datetime
-from textwrap import dedent
-from unittest import SkipTest
 
 from freezegun import freeze_time
 
 
-def test_time_freeze_coroutine():
-    if not asyncio:
-        raise SkipTest('asyncio required')
-
+def test_datetime_in_coroutine():
     @freeze_time('1970-01-01')
     async def frozen_coroutine():
         assert datetime.date.today() == datetime.date(1970, 1, 1)