Revert "Add a new async iterable select() function"

Sadly the generator approach doesn't work because of limitations in how
Python handles the `yield` statement and breaking out from a `async for`
for loop (actually this applies to sync generators too).

When the loop is broken, the control is never passed back to the async
genreator, so the `finally` block is never executed (at least not until
the end of the program, where dangling generators are cleared).

Because of this, we will need to use a class instead, and to make it
easy to guarantee avoiding leaking resources (tasks), we make it an
async context managaer and an async iterator.

This reverts commit 2764d01.

Example:

```python
import asyncio
from typing import AsyncIterator

was_gen_finalized: bool = False

async def gen() -> AsyncIterator[int]:
    try:
        print("gen")
        yield 1
    finally:
        global was_gen_finalized
        was_gen_finalized = True
        print("gen finally")

async def main() -> None:
    global was_gen_finalized
    print("1. without break")
    async for i in gen():
        print(i)
    print(f"  end of loop: {was_gen_finalized=}")
    was_gen_finalized = False

    print("------------------")
    print("2. with break")
    async for i in gen():
        print(i)
        break

    print(f"  end of loop: {was_gen_finalized=}")
    was_gen_finalized = False

    print("------------------")
    print("2. with exception")
    try:
        async for i in gen():
            print(i)
            raise Exception("Interrupted by exception")
    except Exception:
        pass

    print(f"  end of loop: {was_gen_finalized=}")
    was_gen_finalized = False

    print()
    print("END of main")

asyncio.run(main())
print(f"END of asyncio.run(): {was_gen_finalized=}")
```

This program prints:
```
1. without break
gen
1
gen finally
  end of loop: was_gen_finalized=True
------------------
2. with break
gen
1
  end of loop: was_gen_finalized=False
------------------
2. with exception
gen
1
  end of loop: was_gen_finalized=False

END of main
gen finally
gen finally
END of asyncio.run(): was_gen_finalized=True
```

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

Author: llucax

src/frequenz/channels/util/__init__.py

@@ -26,24 +26,13 @@
 * [Select][frequenz.channels.util.Select]: A helper to select the next
   available message for each [receiver][frequenz.channels.Receiver] in a group
   of receivers.
-
-* [select][frequenz.channels.util.select]: A function to iterate over a group
-  of [receivers][frequenz.channels.Receiver] and select the next available value.
 """
 
 from ._event import Event
 from ._file_watcher import FileWatcher
 from ._merge import Merge
 from ._merge_named import MergeNamed
-from ._select import (
-    Select,
-    Selected,
-    SelectError,
-    SelectErrorGroup,
-    UnhandledSelectedError,
-    select,
-    selected_from,
-)
+from ._select import Select
 from ._timer import (
     MissedTickPolicy,
     SkipMissedAndDrift,
@@ -58,15 +47,9 @@
     "Merge",
     "MergeNamed",
     "MissedTickPolicy",
+    "Timer",
     "Select",
-    "SelectError",
-    "SelectErrorGroup",
-    "Selected",
     "SkipMissedAndDrift",
     "SkipMissedAndResync",
-    "Timer",
     "TriggerAllMissed",
-    "UnhandledSelectedError",
-    "select",
-    "selected_from",
 ]