Pin Markupsafe until we are able to upgrade Flask/Jinja

Markupsafe 2.1.0 breaks with error: import name 'soft_unicode' from 'markupsafe'
This should be removed when either this issue is closed:
pallets/markupsafe#284
or when we will be able to upgrade JINJA to newer version (currently limited due to Flask and
Flask Application Builder)

apache/airflow#21664

setup.cfg

@@ -26,6 +26,13 @@ install_requires =
     gcloud-aio-storage
     gcloud-aio-bigquery
     typing_extensions; python_version < "3.8"
+    # Markupsafe 2.1.0 breaks with error: import name 'soft_unicode' from 'markupsafe'.
+    # This should be removed when either this issue is closed:
+    # https://github.com/pallets/markupsafe/issues/284
+    # or when we will be able to upgrade JINJA to newer version (currently limited due to Flask and
+    # Flask Application Builder)
+    # https://github.com/apache/airflow/pull/21664/
+    markupsafe>=1.1.1,<2.1.0
 zip_safe = false
 
 [options.extras_require]