Clientele

* Support for HTTP PUT methods
* Headers objects use `exclude_unset` to avoid passing `None` values as headers, which httpx does not support.

Additionally, an async test client is now included in the test suite. It has identical tests to the standard one but uses the async client instead.

Please delete the constants.py file when updating to this version to have new features take affect

* Paths are resolved correctly when generating clients in nested directories.
* `additional_headers()` is now applied to every client, allowing you to set up headers for all requests made by your client.
* When the client cannot match an HTTP response to a return type for the function it will now raise an `http.APIException`. This object will have the `response` attached to it for inspection by the developer.
* `MANIFEST` is now renamed to `MANIFEST.md` and will include install information for Clientele, as well as information on the command used to generate the client.

Examples and documentation now includes a very complex example schema built using [FastAPI](https://fastapi.tiangolo.com/) that offers the following variations:

* Simple request / response (no input just an output)
* A request with a URL/Path parameter.
* Models with `int`, `str`, `list`, `dict`, references to other models, enums, and `list`s of other models and enums.
* A request with query parameters.
* A response model that has optional parameters.
* An HTTP POST request that takes an input model.
* An HTTP POST request that takes path parameters and also an input model.
* An HTTP GET request that requires an HTTP header, and returns it.
* An HTTP GET endpoint that returns the HTTP bearer authorization token (also makes clientele generate the http authentication for this schema).

A huge test suite has been added to the CI pipeline for this project using a copy of the generated client from the schema above.

* `Enums` now inherit from `str` as well so that they serialize to JSON properly. See [this little nugget](https://hultner.se/quickbits/2018-03-12-python-json-serializable-enum.html).

* Correctly use `model_rebuild` for complex schemas where there are nested schemas, his may be necessary when one of the annotations is a ForwardRef which could not be resolved during the initial attempt to build the schema.
* Do not raise for status, instead attempt to return the response if it cannot match a response type.

* Correctly generate lists of nested schema classes
* Correctly build response schemas that are emphemeral (such as when they just return an array of other schemas, or when they have no $ref).