[AIRFLOW-4472] Use json.dumps/loads for templating lineage data (#5253)

jinja2 cannot use dict/lists as templates hence converting
it to json solves this while keeping complexity down.

(cherry picked from commit a6daeb5)

airflow/lineage/datasets.py

@@ -16,6 +16,7 @@
 # KIND, either express or implied.  See the License for the
 # specific language governing permissions and limitations
 # under the License.
+import json
 import six
 
 from typing import List
@@ -62,7 +63,11 @@ def __getattr__(self, attr):
         if attr in self.attributes:
             if self.context:
                 env = Environment()
-                return env.from_string(self._data.get(attr)).render(**self.context)
+                # dump to json here in order to be able to manage dicts and lists
+                rendered = env.from_string(
+                    json.dumps(self._data.get(attr))
+                ).render(**self.context)
+                return json.loads(rendered)
 
             return self._data.get(attr)
 
@@ -82,7 +87,9 @@ def as_dict(self):
         env = Environment()
         if self.context:
             for key, value in six.iteritems(attributes):
-                attributes[key] = env.from_string(value).render(**self.context)
+                attributes[key] = json.loads(
+                    env.from_string(json.dumps(value)).render(**self.context)
+                )
 
         d = {
             "typeName": self.type_name,

airflow/models/baseoperator.py

@@ -428,8 +428,8 @@ def __init__(
         self._log = logging.getLogger("airflow.task.operators")
 
         # lineage
-        self.inlets = []  # type: Iterable[DataSet]
-        self.outlets = []  # type: Iterable[DataSet]
+        self.inlets = []   # type: List[DataSet]
+        self.outlets = []  # type: List[DataSet]
         self.lineage_data = None
 
         self._inlets = {