Add a new Selector class

This new class acts as an async context manager and an async iterator,
and makes sure no dangling tasks are left behind after a select loop is
done.

This class (as the failed `select()` function) is also designed to
replace the current `Select` implementation with the following
improvements:

* Proper type hinting by using the new helper type guard
  `selected_from()`.
* Fixes potential starvation issues.
* Simplifies the interface by providing values one-by-one.
* Guarantees there are no dangling tasks left behind when used as an
  async context manager.

Signed-off-by: Leandro Lucarella <[email protected]>

Author: llucax

src/frequenz/channels/util/__init__.py

@@ -26,13 +26,24 @@
 * [Select][frequenz.channels.util.Select]: A helper to select the next
   available message for each [receiver][frequenz.channels.Receiver] in a group
   of receivers.
+
+* [Selector][frequenz.channels.util.Selector]: A tool to iterate over the values of all
+  [receivers][frequenz.channels.Receiver] as new values become available.
 """
 
 from ._event import Event
 from ._file_watcher import FileWatcher
 from ._merge import Merge
 from ._merge_named import MergeNamed
-from ._select import Select
+from ._select import (
+    Select,
+    Selected,
+    SelectError,
+    SelectErrorGroup,
+    Selector,
+    UnhandledSelectedError,
+    selected_from,
+)
 from ._timer import (
     MissedTickPolicy,
     SkipMissedAndDrift,
@@ -49,7 +60,13 @@
     "MissedTickPolicy",
     "Timer",
     "Select",
+    "SelectError",
+    "SelectErrorGroup",
+    "Selected",
+    "Selector",
     "SkipMissedAndDrift",
     "SkipMissedAndResync",
     "TriggerAllMissed",
+    "UnhandledSelectedError",
+    "selected_from",
 ]