Adding pure python SSL test using asyncio

- Uses thread.capnp
- Follows same format as thread_client.py/thread_server.py and
async_client.py/async_server.py
- Including a basic self signed certificate for testing convenience
- Python 3.7 has a bug cleaning up SSL when using asyncio.run
  https://bugs.python.org/issue36709
  Have a slightly more verbose workaround to do proper cleanup

examples/selfsigned.cert

@@ -0,0 +1,17 @@
+-----BEGIN CERTIFICATE-----
+MIICpDCCAYwCCQDR+CRWUUUdKDANBgkqhkiG9w0BAQsFADAUMRIwEAYDVQQDDAls
+b2NhbGhvc3QwHhcNMTkwOTE3MDczODI2WhcNNDcwMjAyMDczODI2WjAUMRIwEAYD
+VQQDDAlsb2NhbGhvc3QwggEiMA0GCSqGSIb3DQEBAQUAA4IBDwAwggEKAoIBAQCg
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+vWZiM7Ul+9mOPejUMPwlAgMBAAEwDQYJKoZIhvcNAQELBQADggEBAICkFOw0Dd1U
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+0TmTPtjSQKl5PqcS8kQJTED+CnQhqOAv7C68Bpg+x2dSD9VCq81cPeDbfnK6gico
+29qJYUm4RCXMicrzvEwNObx06TQKJb/pWjpl1NAmpFvcz+2MYPL/QTfH/cS5lhgx
+KCe4/kDO0HCueOi2MqBFaO2B0kZxanMoZ2KZe2b/Bp1CTJzXCXNYWQj6QbLzZxaD
+fOp0J8wAo1U=
+-----END CERTIFICATE-----