Unleashclient

* (Minor) Add support for bootstrapping UnleashClient with an initial configuration.
* (Minor) Add flag to disable toggle polling from server. Thanks povilasb!

* (Bugfix) Clarify logging if Unleash server doesn't return feature provisioning (i.e. HTTP 304).

* (Minor) Move UnleashClient logging to the `UnleashClient` logger.
* (Bugfix) Fix logging for UnleashClient.

* (Major) Support new constraint operators.
* (Major) Add cache abstraction. Thanks walison17!
* (Minor) Refactor `unleash-client-python` to modernize tooling (`setuptools_scm` and centralizing tool config in `pyproject.toml`).
* (Minor) Migrate documentation to Sphinx.

* (Bugfix) Fix HTTP 304 handling on `/client/feature` for Gitlab. Thanks janLo!

* (Breaking) Modify client initialization to allow jitter configuration on refresh/background refresh intervals. May break code when parameters to UnleashClient are not used as keyword arguments. Thanks dryobates and jstolarski!
* (Major) If client is already initialized, calling `initialize_client()` again won't re-run initialization.
* (Minor) Support HTTP 304 on `/client/feature` endpoint.
* (Minor) Rename `master` to `main`.
* (Documentation) Document running UnleashClient in uWSGI. Thanks sighphyre!
* (Documentation) Fix links and formatting. Thanks thomasheartman & sighphyre!