Add __all__ to please mypy (#1928)

* Add __all__ to please mypy

mypy complains with "implicit reexport disabled" when importing directly from the main `rq` package, eg.

from rq import Worker

Without adding an `__all__` to `__init__.py`, mypy would expect it this way:

from rq.worker import Worker

This PR does add the `__all__` to `__init__.py` to please mypy.

* Lint

rq/__init__.py

@@ -5,4 +5,19 @@ from .queue import Queue
 from .version import VERSION
 from .worker import SimpleWorker, Worker
 
+__all__ = [
+    "Connection",
+    "get_current_connection",
+    "pop_connection",
+    "push_connection",
+    "Callback",
+    "Retry",
+    "cancel_job",
+    "get_current_job",
+    "requeue_job",
+    "Queue",
+    "SimpleWorker",
+    "Worker",
+]
+
 __version__ = VERSION