tests.yml: stop testing Python 2.7

Python 2.7 will not be supported by GitHub Actions any longer: actions/setup-python#672

We can continue using PyPy 2.7 to test Python 2 until we drop support in tinytag 2.0.0.

.github/workflows/tests.yml

@@ -9,7 +9,7 @@ jobs:
     strategy:
       matrix:
         os: [ubuntu-20.04, macos-latest, windows-latest]
-        python: ['2.7', '3.5', '3.6', '3.7', '3.8', '3.9', '3.10', '3.11', 'pypy-2.7', 'pypy-3.7', 'pypy-3.8', 'pypy-3.9']
+        python: ['3.5', '3.6', '3.7', '3.8', '3.9', '3.10', '3.11', 'pypy-2.7', 'pypy-3.7', 'pypy-3.8', 'pypy-3.9']
     steps:
       - name: Checkout code
         uses: actions/checkout@v3