define type for enivronment dictionaries

nominatim/typing.py

@@ -9,7 +9,7 @@ Type definitions for typing annotations.
 
 Complex type definitions are moved here, to keep the source files readable.
 """
-from typing import Union, TypeVar, TYPE_CHECKING
+from typing import Union, Mapping, TypeVar, TYPE_CHECKING
 
 # Generics varaible names do not confirm to naming styles, ignore globally here.
 # pylint: disable=invalid-name
@@ -21,6 +21,8 @@ if TYPE_CHECKING:
 
 StrPath = Union[str, 'os.PathLike[str]']
 
+SysEnv = Mapping[str, str]
+
 # psycopg2-related types
 
 Query = Union[str, bytes, 'psycopg2.sql.Composable']