Fix test_run_with_aiohttp_not_installed

Somewhere between 3.5.1 and 3.5.2, the aiohttp reimport started
succeeding (in connexion.connexion.cli.run()). It's not clear which
change caused the issue, but it's probably one of:

- aio-libs/aiohttp#3469 (Remove wildcard imports)
- aio-libs/aiohttp#3464 (Don't suppress gunicorn cleanup errors)
- aio-libs/aiohttp#3471 (Refactor workers)
- aio-libs/aiohttp#3500 (Ignore done tasks)

In any case, setting sys.modules['aiohttp'] = None should prevent
reimporting it. See: https://stackoverflow.com/a/1350574

I successfully tested locally on py37 with aiohttp 3.5.1 and 3.5.2.

tests/test_cli.py

@@ -262,6 +262,7 @@ def test_run_with_wsgi_containers(mock_app_run, spec_file):
 def test_run_with_aiohttp_not_installed(mock_app_run, spec_file):
     import sys
     aiohttp_bkp = sys.modules.pop('aiohttp', None)
+    sys.modules['aiohttp'] = None
 
     runner = CliRunner()