beancount_types: OptionsMap is a dict.

While mutating it is *probably* not a good idea, I've gotten to a point
where I need to do that (to update a value), so just admit the truth here.

conservancy_beancount/beancount_types.py

@@ -13,6 +13,7 @@ import beancount.core.data as bc_data
 from typing import (
     TYPE_CHECKING,
     Any,
+    Dict,
     FrozenSet,
     Iterable,
     List,
@@ -74,5 +75,5 @@ ALL_DIRECTIVES: FrozenSet[Type[Directive]] = frozenset([
 
 Entries = List[Directive]
 Errors = List[Union[Error, 'errormod.Error']]
-OptionsMap = Mapping[str, Any]
+OptionsMap = Dict[str, Any]
 LoadResult = Tuple[Entries, Errors, OptionsMap]