Heksher

This version is only compatible with heksher 0.5.0 and above.
Added
* Added support for setting alias in declaration
* `__version__` is now a module-level attribute
* documentation site
* HeksherEnum, HeksherFlags, HeksherMapping, and HeksherSequence are now public, and are the
preferred way to create complex setting types.
* client.get_settings() also includes the setting's version and alias
* setting.on_coerce, a callable to use if value coercion occurs.
* type coercion and rejection is now supported
* added overridable callback methods on_update_error and on_update_success to Async and
Thread clients.
Removed
* stub rules no longer accept `None` feature values
Changed
* only the new heksher API is now supported
* stub rules now don't have to all have the same context features.
* the server's default value is now respected over the local default value.
* validators now accept the rule object as retrieved from the server
* Each setting now only requires its configurable features to be specified.
* If an async client fails startup or reload, it raises an error.
* default values are now required for settings.
Deprecated
* Creating settings with an alias type, enum type, or flags type is deprecated. use HeksherEnum, HeksherFlags,
HeksherMapping, or HeksherSequence instead.
* using ``patch`` for stub clients is now deprecated. Prefer to set the ``rules`` attribute instead.
* ``AsyncContextManagerMixin.close`` is now deprecated. Use ``AsyncContextManagerMixin.aclose`` instead.

Added
* Added add_validator method for Setting; Validators added to each Setting will be called by order they were added
(each validator will receive value of previous validator); Validators will be called each time the Setting is updated
* Added close checks to async client exit
Changed
* Changing main client between similar clients that track the same contexts is now possible, but not recommended. Changed for testing purposes.

Changed
* Changed httpx version limitation in poetry.toml to `*`

Added
* Added get_settings method to thread and async clients
Internal
* Changed linters to mypy and isort
Fixed
* Added headers to httpx.AsyncClient and httpx.Client, that supports content in body to be json
* Setting.metadata default value is now an empty dict

Fixed
* Removed redundant warning from `subclasses.py` when using `TRACK_ALL` feature.
* Cached time was time-zone dependant causing cache issues.
Internal
* `poetry.lock`, `.coverage`, `pycache`, `coverage.xml` to gitignore
* Make `scripts` files executable (+x) and add shebang to the header.

Fixed
* Send metadata on settings declaration.