Optimize `contextlib.contextmanager` to use `for` loops instead of `next`

[brandtbucher]

The implementation of contextlib.contextmanager currently advances the wrapped generator using explicit next calls.

Replacing these with equivalent for loops allows the interpreter to take advantage of existing iterator-loop optimizations:

• Generator iteration in for loops can be specialized more effectively than direct next calls.
• The interpreter can avoid constructing and raising StopIteration exceptions in the common case where loop termination is expected.
• This is also more JIT-friendly, since iteration is expressed using a common language construct rather than explicit exception-driven control flow.

Local microbenchmarks of entering and exiting a context manager repeatedly show measurable reductions in the overhead introduced by _GeneratorContextManager.

This issue tracks updating the implementation to use for loops where appropriate.

Linked PRs

• GH-151903: Use for instead of next in contextlib.contextmanager #141275