Temporarily pin xmltodict to 0.12.0 to fix main failure

The xmltodict 0,13.0 breaks some tests and likely 0.13.0 is buggy
as the error is ValueError: Malformatted input.

We pin it to 0.12.0 to fix the main failing.

Related: apache#23576

setup.py

@@ -200,6 +200,9 @@ def write_version(filename: str = os.path.join(*[my_dir, "airflow", "git_version
     pandas_requirement,
     'mypy-boto3-rds>=1.21.0',
     'mypy-boto3-redshift-data>=1.21.0',
+    # XML to dict 0.13.0 breaks some EMR tests
+    # It should be removed once we solve https://github.com/apache/airflow/issues/23576
+    'xmltodict<0.13.0',
 ]
 apache_beam = [
     'apache-beam>=2.33.0',