ci: no python-2.7 in setup-python

Despite using a pinned hash for the setup-python v4 action, it has
nevertheless changed under us so that Python 2.7 is no longer
supported.

See actions/setup-python#672

Drop 2.7 from the matrix and just rely on the pypy27 entry for
checking Python 2.x builds.

.github/workflows/ci.yml

@@ -9,7 +9,7 @@ jobs:
     runs-on: ubuntu-20.04
     strategy:
       matrix:
-        python-version: [2.7, 3.5, 3.6, 3.7, 3.8, 3.9, "3.10", 3.11, pypy2.7, pypy3.9]
+        python-version: [3.5, 3.6, 3.7, 3.8, 3.9, "3.10", 3.11, pypy2.7, pypy3.9]
 
     steps:
     - uses: actions/checkout@ac593985615ec2ede58e132d2e21d2b1cbd6127c # v3