Add better typings plus test for transaction.atomic.

- All cases are handled, including bare decorator (@transaction.atomic).
- Decorated function's signature is preserved when type-checking.

django-stubs/db/transaction.pyi

@@ -1,5 +1,4 @@
-from contextlib import ContextDecorator
-from typing import Any, Callable, Optional, Union, Iterator, overload, ContextManager
+from typing import Any, Callable, Optional, overload, TypeVar
 
 from django.db import ProgrammingError
 
@@ -18,19 +17,23 @@ def get_rollback(using: None = ...) -> bool: ...
 def set_rollback(rollback: bool, using: Optional[str] = ...) -> None: ...
 def on_commit(func: Callable, using: None = ...) -> None: ...
 
-class Atomic(ContextDecorator):
+_C = TypeVar("_C", bound=Callable)  # Any callable
+
+# Don't inherit from ContextDecorator, so we can provide a more specific signature for __call__
+class Atomic:
     using: Optional[str] = ...
     savepoint: bool = ...
     def __init__(self, using: Optional[str], savepoint: bool) -> None: ...
+    # When decorating, return the decorated function as-is, rather than clobbering it as ContextDecorator does.
+    def __call__(self, func: _C) -> _C: ...
     def __enter__(self) -> None: ...
     def __exit__(self, exc_type: None, exc_value: None, traceback: None) -> None: ...
 
+# Bare decorator
 @overload
-def atomic() -> Atomic: ...
+def atomic(using: _C) -> _C: ...
+
+# Decorator or context-manager with parameters
 @overload
-def atomic(using: Optional[str] = ...,) -> ContextManager[Atomic]: ...
-@overload
-def atomic(using: Callable = ...) -> Callable: ...
-@overload
-def atomic(using: Optional[str] = ..., savepoint: bool = ...) -> ContextManager[Atomic]: ...
+def atomic(using: Optional[str] = None, savepoint: bool = True) -> Atomic: ...
 def non_atomic_requests(using: Callable = ...) -> Callable: ...