gh-151903: Optimize context managers by replacing next() with for loops

[tmvalijib24]

Fixes #151903

Description

This PR optimizes context managers by replacing explicit next() and anext() iteration calls with structured for and async for loops within _GeneratorContextManager and _AsyncGeneratorContextManager inside Lib/contextlib.py.

Why this helps:

• Avoids the overhead of creating, raising, and catching StopIteration or StopAsyncIteration exceptions on successful context blocks.
• Allows the interpreter to leverage bytecode specialization for generator iteration.

Tests

• Ran and verified locally via ./python -m test test_contextlib (all tests passed successfully).

• Issue: Optimize contextlib.contextmanager to use for loops instead of next #151903

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[python-cla-bot]

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[StanFromIreland]

The issue already has an open PR (by Brandt himself), closing in favour of the existing work.

[tmvalijib24]

Hi @StanFromIreland, I am New to Open Source and interested in Contributing for Cpython, is there any discord channel in which I can connect to the maintainers and contributors?

[StanFromIreland]

I'm afraid there isn't, I suggest reading the devguide.