remove httpx constraint as new respx versions are already fixed

[rooterkyberian]

What

Removed temporary httpx ver constraint as lundberg/respx#277 was already resolved in respx

Checklist

• Contains customer facing changes? Including API/behavior changes
  • no API changes. It just allows installing newer httpx
• Did you add sufficient unit test and/or integration test coverage for this PR?
  • the issue was affecting already existing tests so this is covered already

References

lundberg/respx#277

Test & Review

• unittests

[confluent-cla-assistant]

🎉 All Contributor License Agreements have been signed. Ready to merge.
✅ rooterkyberian
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[rayokota]

Thanks @rooterkyberian , LGTM