Rename the util._select module to util._selector

This is to match the main contents of the module, the `Selector` class.

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

Author: llucax

src/frequenz/channels/util/__init__.py

@@ -31,7 +31,7 @@
 from ._file_watcher import FileWatcher
 from ._merge import Merge
 from ._merge_named import MergeNamed
-from ._select import (
+from ._selector import (
     Selected,
     SelectError,
     SelectErrorGroup,