bpo-23819: Get rid of assert statements in test_asyncio (GH-30212)

To keep checks even if run tests with optimized Python.

Either use special assertion methods like assertEqual() or
raise an AssertionError explicitly.

Lib/test/test_asyncio/test_unix_events.py

@@ -34,7 +34,8 @@ def EXITCODE(exitcode):
 
 
 def SIGNAL(signum):
-    assert 1 <= signum <= 68
+    if not 1 <= signum <= 68:
+        raise AssertionError(f'invalid signum {signum}')
     return 32768 - signum