[workflow] Cache deps, drop python 3.9 testing

- Dependency caching wasn't working because the dependency path wasn't
right, so fix that.
- Python 3.9 doesn't work for Windows or Mac, and few
users will be on linux using 3.9 so drop it to save time.

.github/workflows/build.yml

@@ -16,13 +16,8 @@ jobs:
   lint-and-test:
     strategy:
       matrix:
-        python-version: ["3.9", "3.10", "3.11"]
+        python-version: ["3.10", "3.11"]
         os: [ubuntu-latest, macos-latest, windows-latest]
-        exclude:
-          - os: macos-latest
-            python-version: "3.9"
-          - os: windows-latest
-            python-version: "3.9"
       fail-fast: false
 
     runs-on: ${{ matrix.os }}
@@ -36,9 +31,9 @@ jobs:
       with:
         python-version: ${{ matrix.python-version }}
         cache: "pip"
+        cache-dependency-path: requirements*.txt
     - name: Install dependencies
       run: |
-        python -m pip install --upgrade pip
         python -m pip install -r requirements-dev.txt
     - name: Lint with flake8
       run: |