Disables --warn-unused-ignore flag for mypy (#10880)

There is a problem with MyPy's implementation of
--warn-unused-ignore flag, that depending on it's incremental
or full run it will sometimes throw an "unused ignore" error
(entirely randomly it seems). The problem is described
(but only workarounded) in
python/mypy#2960.

The workaround is to disable --warn-unused-ignore flag.
There is little harm in having unused ignores and we can
clean them up from time to time easily.

Author: potiuk

airflow/models/dagrun.py

@@ -69,7 +69,7 @@ class DagRun(Base, LoggingMixin):
 
     task_instances = relationship(
         TI,
-        primaryjoin=and_(TI.dag_id == dag_id, TI.execution_date == execution_date),
+        primaryjoin=and_(TI.dag_id == dag_id, TI.execution_date == execution_date),  # type: ignore
         foreign_keys=(dag_id, execution_date),
         backref=backref('dag_run', uselist=False),
     )

setup.cfg

@@ -59,7 +59,7 @@ packages = airflow
 ignore_missing_imports = True
 no_implicit_optional = True
 warn_redundant_casts = True
-warn_unused_ignores = True
+warn_unused_ignores = False
 plugins =
   airflow.mypy.plugin.decorators
 pretty = True