Make decorator functions transparent to Mypy (#306)

By declaring return type as -> Callable[[_C], _C], Mypy can infer that
the decorated function has also the same arguments and return type as
the original.

View functions are constrained to return HttpResponseBase (or any
subclass of it).

Also added typecheck test coverage to most of the cases.

test-data/typecheck/db/test_transaction.yml

@@ -0,0 +1,28 @@
+-   case: atomic_bare
+    main: |
+      from django.db.transaction import atomic
+      @atomic
+      def func(x: int) -> list: ...
+      reveal_type(func)  # N: Revealed type is 'def (x: builtins.int) -> builtins.list[Any]'
+-   case: atomic_args
+    main: |
+      from django.db.transaction import atomic
+      @atomic(using='bla', savepoint=False)
+      def func(x: int) -> list: ...
+      reveal_type(func)  # N: Revealed type is 'def (x: builtins.int) -> builtins.list[Any]'
+-   case: non_atomic_requests_bare
+    main: |
+      from django.db.transaction import non_atomic_requests
+      @non_atomic_requests
+      def view_func(request): ...
+      reveal_type(view_func)  # N: Revealed type is 'def (request: Any) -> Any'
+
+-   case: non_atomic_requests_args
+    main: |
+      from django.http.request import HttpRequest
+      from django.http.response import HttpResponse
+      from django.db.transaction import non_atomic_requests
+      @non_atomic_requests
+      def view_func(request: HttpRequest, arg: str) -> HttpResponse: ...
+      reveal_type(view_func)  # N: Revealed type is 'def (request: django.http.request.HttpRequest, arg: builtins.str) -> django.http.response.HttpResponse'
+