Better asyncio handling in Python <3.7 using threading

[caronc]

Description:

Related issue (if applicable): nvidia-sniper/103

Apprise leverages asyncio right out of the box (by default), which works perfect in most situations. Asyncio significantly evolved once Python v3.7 was released. The problem is that not everyone is using Python v3.7, and not everyone has adapted their tool sets to use the new asyncio approaches yet.

Users of Python v3.6 and lower can encounter an error when using Apprise if the application it was built under leverages threading. By default there is no event_loop that exists within a Python thread. This pull request attempts to detect these situations so that it can spin up a new event loop (similar to how Python v3.7+ would) making the Apprise experience/integration more developer/user friendly. This PR also eliminates any potential RuntimeError from occurring on these older versions of Python.

Checklist

• The code change is tested and works locally.
• There is no commented out code in this PR.
• No lint errors (use flake8)
• 100% test coverage