def _merge_pool_kwargs(
        self, override: dict[str, typing.Any] | None
    ) -> dict[str, typing.Any]:
        """
        Merge a dictionary of override values for self.connection_pool_kw.

        This does not modify self.connection_pool_kw and returns a new dict.
        Any keys in the override dictionary with a value of ``None`` are
        removed from the merged dictionary.
        """
        base_pool_kwargs = self.connection_pool_kw.copy()
        if override:
            for key, value in override.items():
                if value is None:
                    try:
                        del base_pool_kwargs[key]
                    except KeyError:
                        pass
                else:
                    base_pool_kwargs[key] = value
        return base_pool_kwargs

        Merge a dictionary of override values for self.connection_pool_kw.

        This does not modify self.connection_pool_kw and returns a new dict.
        Any keys in the override dictionary with a value of ``None`` are
        removed from the merged dictionary.
        
src/urllib3/poolmanager.py