v3.0.0: Modernize for latest Starlette, drop EOL Pythons

- Bump version to 3.0.0
- Replace deprecated starlette.middleware.wsgi with a2wsgi
- Pin starlette[full]>=0.40, add a2wsgi dependency
- Bump minimum Python to 3.9, drop 3.7/3.8 support
- Remove whitenoise conditional (Python <3.8 no longer supported)
- Clean up CI matrix: drop old Python versions and OS exclusions
- Fix deprecated httpx TestClient usage in tests (data= -> content=, per-request cookies)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.github/workflows/test.yaml

@@ -20,15 +20,11 @@ jobs:
       fail-fast: false
       matrix:
         os: [
-          "ubuntu-20.04",
-          "macos-13",
+          "ubuntu-latest",
           "macos-latest",
           "windows-latest",
         ]
         python-version: [
-          "3.6",
-          "3.7",
-          "3.8",
           "3.9",
           "3.10",
           "3.11",
@@ -36,33 +32,6 @@ jobs:
           "3.13",
           "pypy3.10",
         ]
-
-        exclude:
-
-          # Exclude test matrix slots that are no longer supported by GHA runners.
-          - os: 'ubuntu-20.04'
-            python-version: '3.6'
-          - os: 'macos-latest'
-            python-version: '3.6'
-          - os: 'macos-latest'
-            python-version: '3.7'
-          - os: 'macos-latest'
-            python-version: '3.8'
-          - os: 'macos-latest'
-            python-version: '3.9'
-          - os: 'macos-latest'
-            python-version: '3.10'
-
-          # Exclude Python 3.7 on Windows, because GHA fails on it.
-          #
-          #   SyntaxError: Non-UTF-8 code starting with '\x83' in file
-          #   C:\hostedtoolcache\windows\Python\3.7.9\x64\Scripts\poe.exe
-          #   on line 2, but no encoding declared;
-          #   see http://python.org/dev/peps/pep-0263/ for details
-          #
-          # https://github.com/kennethreitz/responder/actions/runs/11526258626/job/32090071392?pr=546#step:6:73
-          - os: 'windows-latest'
-            python-version: '3.7'
     env:
       UV_SYSTEM_PYTHON: true
 
@@ -77,9 +46,6 @@ jobs:
       with:
         python-version: ${{ matrix.python-version }}
         architecture: x64
-        cache: 'pip'
-        cache-dependency-path: |
-          pyproject.toml
 
     - name: Set up uv
       uses: astral-sh/setup-uv@v5
@@ -115,9 +81,6 @@ jobs:
       with:
         python-version: ${{ matrix.python-version }}
         architecture: x64
-        cache: 'pip'
-        cache-dependency-path: |
-          pyproject.toml
 
     - name: Set up uv
       uses: astral-sh/setup-uv@v5