Be able to select mqtt:tls_version for Python < 3.6 (#6442)

* Be able to select tls_version

* This test should always assert this value, not only in 3.6

* Disable linting on future property (py36)

* Only allow TLS 1.0, 1.1 and 1.2

* Fix line length issue

* Fix check config tests

* Allow auto as a TLS version

tests/scripts/test_check_config.py

@@ -108,6 +108,7 @@ class TestCheckConfig(unittest.TestCase):
                     'protocol': '3.1.1',
                     'discovery': False,
                     'discovery_prefix': 'homeassistant',
+                    'tls_version': 'auto',
                 },
                  'light': []},
                 res['components']