Pin httpx dependency so spleeter follows redirects for model downloading

[microtherion]

At the moment, spleeter model downloads are failing, at least for me, because:

• spleeter models when downloaded from github go through a redirection.
• spleeter-web specifies a httpx dependency of 0.26.0.
• As of 0.20.0, httpx no longer follows redirects by default.
• spleeter fails to explicitly request following redirects.

This issue has been called out in an unmerged spleeter PR: deezer/spleeter#808 but that has not been merged for a long time.

One possible solution might be to downgrade the httpx dependency to < 0.20.0, but I don't know what other repercussions this could have. The other solution, which I've chosen here, is to fix the spleeter code with a patch applied at container build time.
After discussion of alternatives with @JeffreyCA, this PR pins httpx at 0.19.0

[JeffreyCA]

Thanks for the fix and catching this! This was actually a regression in the recent release and the version of httpx being installed before was 0.19.0. Could we downgrade it instead in requirements.txt?

[microtherion]

Sure, I'll try that. I assumed there was a reason for the version bump, so I did not investigate that avenue. But should there be safeguards in place to avoid a repetition of this problem?

[JeffreyCA]

There's a dependabot.yml we can add httpx to

[microtherion]

I verified that pinning httpx to 0.19.0 works. I hope the dependabot change is correct — I have no experience with that and would not know how to test it.

[JeffreyCA]

Perfect, thanks again!