[python] Remove useless dev dependency from generator test requirements (#10193)

## Summary
- remove an unnecessary dependency from generator test dev requirements
- add a changelog entry for this change

## Files changed
- .chronus/changes/python-remove-useless-dev-dep-2026-2-30-12-41-26.md
- packages/http-client-python/generator/test/dev_requirements.txt

## Validation
- no runtime behavior changes expected; dependency list cleanup only

Author: msyyc

.chronus/changes/python-remove-useless-dev-dep-2026-2-30-12-41-26.md

@@ -0,0 +1,7 @@
+---
+changeKind: internal
+packages:
+  - "@typespec/http-client-python"
+---
+
+remove useless dependency in dev requirements.txt

packages/http-client-python/generator/test/dev_requirements.txt

@@ -1,7 +1,6 @@
 -r ../../eng/scripts/ci/dev_requirements.txt
 aiohttp
 pytest-asyncio==0.14.0
-requests==2.32.2
 sphinx==8.2.0
 sphinx_rtd_theme==3.0.2
 myst_parser==4.0.1