Lazy Jinja2 context

[uranusjr]

Fix #20213. To work around Jinja2 eagerly resolving context into a new dict, we implement a custom copy interface, and use lower level Jinja2 functions to render templates (because the higher level Template.render() simply calls dict(**data) which is not overridable).

Tests are added to ensure that

1. Rendering a template with deprecated variable emits a deprecation warning.
2. Rendering a template without deprecated variable does not emit a deprecation warning.

[github-actions]

The PR most likely needs to run full matrix of tests because it modifies parts of the core of Airflow. However, committers might decide to merge it quickly and take the risk. If they don't merge it quickly - please rebase it to the latest main at your convenience, or amend the last commit of the PR, and push it with --force-with-lease.

[uranusjr]

Don’t merge this yet, there’s another case we need to fix (see comment in linked issue). I’m also thinking I should enable this deprecation-as-error globally in case there are more things we have not catch.

[uranusjr]

Alright I’m done.

[jedcunningham]

Hmm, I'm not seeing a deprecation warning with this:

from datetime import datetime

from airflow import DAG
from airflow.operators.bash import BashOperator

with DAG(
    dag_id="no-context", schedule_interval=None, start_date=datetime(2021, 12, 10)
) as dag:
    BashOperator(task_id="sleep", bash_command="echo 'Hello! {{ execution_date }}'")

[uranusjr]

tests/executors/test_dask_executor.py::TestDaskExecutor::test_dask_executor_functions:
ValueError: The futures should have finished; there is probably an error communicating with the Dask cluster.

Only one job failing with this, likely a network issue.