ci: Update the list of supported python versions

Recently, Amaranth HDL dropped support for Python 3.7.
Since this version of the interpreter is more than 5 years old,
we may also drop the support for it.

Signed-off-by: Robert Winkler <rwinkler@antmicro.com>

.github/workflows/ci.yml

@@ -8,7 +8,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        python-version: [3.7, 3.8, 3.9]
+        python-version: ["3.8", "3.9", "3.10", "3.11"]
 
     steps:
     - uses: actions/checkout@v3