[AIRFLOW-XXX] Fix PythonVirtualenvOperator tests (apache#3968)

The recent update to the CI image changed the default
python from python2 to python3. The PythonVirtualenvOperator
tests expected python2 as default and fail due to
serialisation errors.

scripts/ci/run-ci.sh

@@ -32,7 +32,7 @@ fi
 if [[ $PYTHON_VERSION == '3' ]]; then
   PIP=pip3
 else
-  PIP=pip
+  PIP=pip2
 fi
 
 sudo $PIP install --upgrade pip

tests/operators/test_virtualenv_operator.py

@@ -55,9 +55,10 @@ def setUp(self):
             schedule_interval=INTERVAL)
         self.addCleanup(self.dag.clear)
 
-    def _run_as_operator(self, fn, **kwargs):
+    def _run_as_operator(self, fn, python_version=sys.version_info[0], **kwargs):
         task = PythonVirtualenvOperator(
             python_callable=fn,
+            python_version=python_version,
             task_id='task',
             dag=self.dag,
             **kwargs)