httpx deprecation warnings

[natan-abolafya]

Hello.

We've been getting this warning quite a lot in our tests for some time

 ....\.venv\Lib\site-packages\httpx\_models.py:408: DeprecationWarning: Use 'content=<...>' to upload raw bytes/text content.
    headers, stream = encode_request(

I kept looking for a bad use on our code and couldn't find it and assumed it was a false report. Until today when I decided to debug it and found out that it's python-keycloak which is causing this warning. More info at https://www.python-httpx.org/compatibility/#request-content

I would be happy to make a PR unless there is a need to keep it this way, such as httpx backwards compatibility.