Try to support Python 3.9 for signer

This should start failing on CI since we do use some 3.10 features
currently.

This only tests signer on python 3.9: repository has an annoying
dependency issue that would require downgrading urllib3...

.github/workflows/ci.yml

@@ -15,7 +15,13 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
+        pyversion: ['3.12']
         toxenv: [lint-signer, lint-repo, test-signer, test-repo, test-e2e]
+        include:
+          - pyversion: '3.9'
+            toxenv: lint-signer
+          - pyversion: '3.9'
+            toxenv: test-signer
     env:
       TOXENV: ${{ matrix.toxenv }}
 
@@ -24,7 +30,7 @@ jobs:
 
     - uses: actions/setup-python@f677139bbe7f9c59b41e40162b753c062f5d49a3 # v5.2.0
       with:
-        python-version: '3.12'
+        python-version: ${{ matrix.pyversion }}
         cache: 'pip'
         cache-dependency-path: |
           signer/pyproject.toml

signer/pyproject.toml

@@ -13,7 +13,7 @@ dependencies = [
   "tuf ~= 3.1",
   "click ~= 8.1",
 ]
-requires-python = ">=3.10"
+requires-python = ">=3.9"
 dynamic = ["version"]
 
 [project.scripts]