Fix some new-MyPy typing lints.

In one case MyPy got smarter in a way we hoped it one, while in two
others it started complaining differently about a case where we were
deliberately playing fast and loose with types.

python/lsst/daf/butler/_named.py

@@ -272,7 +272,7 @@ def freeze(self) -> NamedKeyMapping[K, V]:
             to a new variable (and considering any previous references
             invalidated) should allow for more accurate static type checking.
         """
-        if not isinstance(self._dict, MappingProxyType):
+        if not isinstance(self._dict, MappingProxyType):  # type: ignore
             self._dict = MappingProxyType(self._dict)  # type: ignore
         return self
 
@@ -592,7 +592,7 @@ def freeze(self) -> NamedValueAbstractSet[K]:
             to a new variable (and considering any previous references
             invalidated) should allow for more accurate static type checking.
         """
-        if not isinstance(self._mapping, MappingProxyType):
+        if not isinstance(self._mapping, MappingProxyType):  # type: ignore
             self._mapping = MappingProxyType(self._mapping)  # type: ignore
         return self
 

python/lsst/daf/butler/persistence_context.py

@@ -33,7 +33,7 @@
 import uuid
 from collections.abc import Callable, Hashable
 from contextvars import Context, ContextVar, Token, copy_context
-from typing import TYPE_CHECKING, ParamSpec, TypeVar, cast
+from typing import TYPE_CHECKING, ParamSpec, TypeVar
 
 if TYPE_CHECKING:
     from ._dataset_ref import DatasetRef
@@ -194,8 +194,6 @@ def run(self, function: Callable[_Q, _T], *args: _Q.args, **kwargs: _Q.kwargs) -
         """
         self._ctx = copy_context()
         # Type checkers seem to have trouble with a second layer nesting of
-        # parameter specs in callables, so ignore the call here and explicitly
-        # cast the result as we know this is exactly what the return type will
-        # be.
+        # parameter specs in callables, so ignore the call here.
         result = self._ctx.run(self._functionRunner, function, *args, **kwargs)  # type: ignore
-        return cast(_T, result)
+        return result