Re-organize directory structure (#4971)

See discussion in #2491

Co-authored-by: Ivan Levkivskyi <ilevkivskyi@dropbox.com>

stdlib/dbm/gnu.pyi

@@ -0,0 +1,35 @@
+from types import TracebackType
+from typing import List, Optional, Type, TypeVar, Union, overload
+
+_T = TypeVar("_T")
+_KeyType = Union[str, bytes]
+_ValueType = Union[str, bytes]
+
+class error(OSError): ...
+
+# Actual typename gdbm, not exposed by the implementation
+class _gdbm:
+    def firstkey(self) -> Optional[bytes]: ...
+    def nextkey(self, key: _KeyType) -> Optional[bytes]: ...
+    def reorganize(self) -> None: ...
+    def sync(self) -> None: ...
+    def close(self) -> None: ...
+    def __getitem__(self, item: _KeyType) -> bytes: ...
+    def __setitem__(self, key: _KeyType, value: _ValueType) -> None: ...
+    def __delitem__(self, key: _KeyType) -> None: ...
+    def __len__(self) -> int: ...
+    def __enter__(self) -> _gdbm: ...
+    def __exit__(
+        self, exc_type: Optional[Type[BaseException]], exc_val: Optional[BaseException], exc_tb: Optional[TracebackType]
+    ) -> None: ...
+    @overload
+    def get(self, k: _KeyType) -> Optional[bytes]: ...
+    @overload
+    def get(self, k: _KeyType, default: Union[bytes, _T]) -> Union[bytes, _T]: ...
+    def keys(self) -> List[bytes]: ...
+    def setdefault(self, k: _KeyType, default: _ValueType = ...) -> bytes: ...
+    # Don't exist at runtime
+    __new__: None  # type: ignore
+    __init__: None  # type: ignore
+
+def open(__filename: str, __flags: str = ..., __mode: int = ...) -> _gdbm: ...