chore(typing): add more descriptive typing to Context (#2226)

* chore(typing): add more descriptive typing to `Context`

* chore(typing): define only stubs when under `TYPE_CHECKING`

falcon/util/structures.py

@@ -37,6 +37,7 @@
 from typing import KeysView
 from typing import Optional
 from typing import Tuple
+from typing import TYPE_CHECKING
 from typing import ValuesView
 
 
@@ -141,6 +142,19 @@ class Context:
     True
     """
 
+    # NOTE(vytas): Define synthetic attr access methods (under TYPE_CHECKING)
+    #   merely to let mypy know this is a namespace object.
+    if TYPE_CHECKING:
+
+        def __getattr__(self, name: str) -> Any:
+            ...
+
+        def __setattr__(self, name: str, value: Any) -> None:
+            ...
+
+        def __delattr__(self, name: str) -> None:
+            ...
+
     def __contains__(self, key: str) -> bool:
         return self.__dict__.__contains__(key)