Remove usage of "run_until_complete" (#16617)

* De-run_forever()-ization

* Use asyncio.run (or our own implementation on Python <3.7)
* hass.start is only used by tests
* setup_and_run_hass() is now async
* Add "main" async hass.run method
* move SIGINT handling to helpers/signal.py
  * add flag to .run to disable hass's signal handlers
* Teach async_start and async_stop to not step on each other
  (more than necessary)

* shorten over-long lines

* restore missing "import asyncio"

* move run_asyncio to homeassistant.util.async_

* LOGGER: warn => warning

* Add "force" flag to async_stop

only useful for testing

* Add 'attrs==18.2.0' to requirements_all.txt

Required for keeping requirements_test_all.txt in sync, where it is in
turn required to prevent auto-downgrading "attrs" during "pip install"

* Fixes for mypy

* Fix "mock_signal" fixture

* Revert mistaken edit

* Flake8 fixes

* mypy fixes

* pylint fix

* Revert adding attrs== to requirements_test*.txt

solved by using "pip -c"

* Rename "run" to "async_run", as per calling conventions

tests/test_bootstrap.py

@@ -22,7 +22,6 @@ _LOGGER = logging.getLogger(__name__)
     'homeassistant.bootstrap.conf_util.process_ha_config_upgrade', Mock())
 @patch('homeassistant.util.location.detect_location_info',
        Mock(return_value=None))
-@patch('homeassistant.bootstrap.async_register_signal_handling', Mock())
 @patch('os.path.isfile', Mock(return_value=True))
 @patch('os.access', Mock(return_value=True))
 @patch('homeassistant.bootstrap.async_enable_logging',
@@ -41,7 +40,6 @@ def test_from_config_file(hass):
 
 
 @patch('homeassistant.bootstrap.async_enable_logging', Mock())
-@patch('homeassistant.bootstrap.async_register_signal_handling', Mock())
 @asyncio.coroutine
 def test_home_assistant_core_config_validation(hass):
     """Test if we pass in wrong information for HA conf."""