Update contextlib.py

[amosonn]

Moved raise from inside try to try..else.
The exception raised, in case the generator doesn't stop after .raise, should be raised in an else block. This is a) more readable, b) saves the catching, checking and reraising of this exception.

[the-knights-who-say-ni]

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[zhangyangyu]

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[ncoghlan]

I agree this is a little clearer than the original, so I went ahead and merged it. There's no functional impact though, so I didn't worry about adding a NEWS entry.

[Mariatta]

I'm backporting this into 3.5 and 3.6 to make the backport for #949 goes smoother.