[ci] Reestablish compatibility with Python 2.7

Does this even make sense? ;]

Author: amotl

.github/workflows/tests.yml

@@ -10,7 +10,7 @@ jobs:
     strategy:
       matrix:
         os: [ubuntu-latest, macos-latest, windows-latest]
-        python-version: ["3.6", "3.7", "3.8", "3.9"]
+        python-version: ["2.7", "3.6", "3.7", "3.8", "3.9"]
         include:
         - os: ubuntu-latest
           path: ~/.cache/pip

tests/test_core.py

@@ -272,7 +272,7 @@ def test_config_bad_functions(caplog):
     with caplog.at_level(logging.DEBUG):
 
         # Bootstrapping the machinery with invalid path to functions file should croak.
-        with pytest.raises(OSError) as excinfo:
+        with pytest.raises(IOError) as excinfo:
             core_bootstrap(configfile=configfile_bad_functions)
 
         error_message = str(excinfo.value)