Fix TypeError due to IntEnum

[tlogemann]

Summary

The issue is described here: #1048

The problem is that at least python 3.12 raises a TypeError for the used IntEnum when using from httpx.

If this is not the intended way of fixing, feel free to close the PR. It was just a quick fix for me.

Checklist

• I understand that this PR may be closed in case there was no previous discussion. (This doesn't apply to typos!)
• I've added a test for each change that was introduced.
• I tried as much as possible to make a single atomic change
• I've updated the documentation accordingly.

[tlogemann]

The TypeError by IntEnum seems not be caused by httpcore, as I also get it from other libraries.