STY: Apply ruff/flake8-pyi rule PYI034

PYI034 `__enter__` methods usually return `self` at runtime

Author: DimitriPapadopoulos

nibabel/openers.py

@@ -22,6 +22,7 @@
     from types import TracebackType
 
     from _typeshed import WriteableBuffer
+    from typing_extensions import Self
 
     ModeRT = ty.Literal['r', 'rt']
     ModeRB = ty.Literal['rb']
@@ -246,7 +247,7 @@ def close_if_mine(self) -> None:
         if self.me_opened:
             self.close()
 
-    def __enter__(self) -> Opener:
+    def __enter__(self) -> Self:
         return self
 
     def __exit__(