change chained maps to MutableMapping; fix #6042

[ramalho]

collections.ChainMap is annotated as a MutableMapping, therefore the mappings it holds must also be MutableMapping.

As it is, operations that update any of the chained mappings are flagged as errors.

This fixes issue #6042

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

pandas (https://github.com/pandas-dev/pandas.git)
+ pandas/compat/chainmap.py:21: error: Redundant cast to "MutableMapping[_KT, _VT]"  [redundant-cast]
+ pandas/compat/chainmap.py:25: error: Redundant cast to "MutableMapping[_KT, _VT]"  [redundant-cast]
+ pandas/compat/chainmap.py:35: error: Redundant cast to "MutableMapping[_KT, _VT]"  [redundant-cast]
+ pandas/core/computation/scope.py:241: error: unused "type: ignore" comment

python-chess (https://github.com/niklasf/python-chess.git)
+ chess/engine.py:1438: error: Argument 1 to "ChainMap" has incompatible type "Mapping[str, Union[str, int, bool, None]]"; expected "MutableMapping[str, Union[str, int, None]]"
+ chess/engine.py:1460: error: Argument 1 to "ChainMap" has incompatible type "Mapping[str, Union[str, int, bool, None]]"; expected "MutableMapping[str, Union[str, int, None]]"
+ chess/engine.py:2351: error: Argument 1 to "ChainMap" has incompatible type "Mapping[str, Union[str, int, bool, None]]"; expected "MutableMapping[str, Union[str, int, None]]"

edgedb (https://github.com/edgedb/edgedb.git)
+ edb/schema/delta.py:1157:38: error: Argument 1 to "ChainMap" has incompatible type "*List[Mapping[Optional[str], str]]"; expected "MutableMapping[Optional[str], str]"