Upgrade uv helper dependencies and add Python 3.9 fallback

Apply the same approach from #15058 to the uv ecosystem:
- Upgrade helper packages to latest versions (pip 26.1.1, poetry 2.4.1, etc.)
- Add requirements-3.9.txt with old 3.9-compatible versions
- Update build script with conditional requirements selection

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: kbukum1

uv/helpers/build

@@ -15,10 +15,21 @@ cp -r \
   "$helpers_dir/lib" \
   "$helpers_dir/run.py" \
   "$helpers_dir/requirements.txt" \
+  "$helpers_dir/requirements-3.9.txt" \
   "$install_dir"
 
 cd "$install_dir"
-PYENV_VERSION=$1 pyenv exec pip3 --disable-pip-version-check install --use-pep517 -r "requirements.txt"
+
+python_version=$1
+# pip 26.x and several other packages require Python >=3.10.
+# Use 3.9-compatible versions for the deprecated Python 3.9 runtime.
+if [[ "$python_version" == 3.9.* ]]; then
+  req_file="requirements-3.9.txt"
+else
+  req_file="requirements.txt"
+fi
+
+PYENV_VERSION=$python_version pyenv exec pip3 --disable-pip-version-check install --use-pep517 -r "$req_file"
 
 # Remove the extra objects added during the previous install. Based on
 # https://github.com/docker-library/python/blob/master/Dockerfile-linux.template

uv/helpers/requirements-3.9.txt

@@ -0,0 +1,15 @@
+# Python 3.9-compatible versions pinned to the last known working set before
+# packages dropped 3.9 support. Python 3.9 reached end-of-life on 2025-10-31.
+pip==24.0
+pip-tools==7.4.1
+flake8==7.1.0
+hashin==1.0.3
+pipenv==2024.0.2
+plette==2.1.0
+poetry==1.8.5
+# tomli is required for Python <3.11 (stdlib tomllib was added in 3.11).
+tomli==2.0.1
+uv==0.11.8
+
+# Some dependencies will only install if Cython is present
+Cython==3.0.10

uv/helpers/requirements.txt

@@ -1,13 +1,13 @@
-pip==24.0
-pip-tools==7.4.1
-flake8==7.1.0
-hashin==1.0.3
-pipenv==2024.0.2
-plette==2.1.0
-poetry==1.8.5
+pip==26.1.1
+pip-tools==7.5.3
+flake8==7.3.0
+hashin==1.0.5
+pipenv==2024.4.1
+plette==2.2.1
+poetry==2.4.1
 # TODO: Replace 3p package `tomli` with 3.11's new stdlib `tomllib` once we drop support for Python 3.10.
-tomli==2.0.1
+tomli==2.4.1
 uv==0.11.8
 
 # Some dependencies will only install if Cython is present
-Cython==3.0.10
+Cython==3.2.4