gh-106751: Optimize SelectSelector.select() for many iteration case

[corona10]

closes: gh-106751

[corona10]

Similar technique can be adopted.

Benchmark

import timeit
import os
from selectors import SelectSelector, EVENT_WRITE, EVENT_READ

select = SelectSelector()
for _ in range(100):
    r, w = os.pipe()
    os.write(w, b"a")
    select.register(r, EVENT_READ)

elapsed = timeit.timeit(
    "selector.select()",
    number=100000,
    globals={"selector": select},
)

print(f"elapsed: {elapsed}")

Result

AS-IS: elapsed -> 2.051669834000222
TO-BE: elapsed -> 1.9723411249997298 (1.04x faster)

[corona10]

cc @eendebakpt

[eendebakpt]

Note: on my system frozenset is slightly slower than set, but the difference is very small

[eendebakpt]

The rw is only used once. You assigned r | w to a variable for readability?