Unleashclient

* (Minor) Only send metrics to API if feature toggle is in-use (i.e. has been resolved to True/False). Thanks fwpheckel!
* (Minor) Remove dangling `variations` reference in favor of `variants` verbiage.

* (Major) Fix bug where loader didn't properly refresh variants. Thanks simenaasland!

* (Minor) Add Python 3.9 support.
* (Minor) Only log errors generated when strategy loading fails once.
* (Minor) Errors submitting metrics will be logged as warnings and not exceptions.
* (Minor) Update apscheduler version to 3.7.0

* (Minor) Better error handling and typo fixes. Thanks vgerak!
* (Minor) Update requests version to 2.25.1.

* (Major) Stop using the `default_value` argument in the `is_enabled()` method (as it can cause counter-intuitive behavior) and add deprecation warning. This argument will be removed in the next major version upgrade!
* We recommend using the `fallback_function` argument instead. If you need a blanket True in case of an exception, you can pass in a lambda like: `lambda x, y: True`.
* (Minor) Add better logging for API errors.
* (Minor) Update requests version to v2.25.0.

**General**
* (Minor) Move CI to Github Actions, add auto-publishing.