Allow using Click 7 with a DeprecationWarning

As long as popular libraries (e.g. Celery) require click 7, depending
on Click 8 in Flask makes it hard to test the latest version (and its
other dependencies) in existing applications.

tests/test_cli.py

@@ -5,6 +5,7 @@ import ssl
 import sys
 import types
 from functools import partial
+from unittest.mock import patch
 
 import click
 import pytest
@@ -21,6 +22,7 @@ from flask.cli import FlaskGroup
 from flask.cli import get_version
 from flask.cli import load_dotenv
 from flask.cli import locate_app
+from flask.cli import main as cli_main
 from flask.cli import NoAppException
 from flask.cli import prepare_import
 from flask.cli import run_command
@@ -660,3 +662,11 @@ def test_cli_empty(app):
 
     result = app.test_cli_runner().invoke(args=["blue", "--help"])
     assert result.exit_code == 2, f"Unexpected success:\n\n{result.output}"
+
+
+def test_click_7_deprecated():
+    with patch("flask.cli.cli"):
+        if int(click.__version__[0]) < 8:
+            pytest.deprecated_call(cli_main, match=".* Click 7 is deprecated")
+        else:
+            cli_main()