Fix api_generator.parser._gen_pairs to work in Python 3.7

[StanislawSwierc]

In Python 3.7 recommendation from PEP 479 was enabled. In the new
behavior StopIteration exceptions raised directly or indirectly in coroutines
and generators are transformed into RuntimeError exceptions.

https://www.python.org/dev/peps/pep-0479/

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[StanislawSwierc]

I've just signed the Contributor License Agreement.

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[lamberta]

Thanks, Stanislaw.
I looks like you signed the CLA using a different email address than the one used for this commit. Can you sign the CLA with this email or re-submit with the other?

[yashk2810]

How about this instead to iter?

@MarkDaoust

zip(items[::2], items[1::2])

[StanislawSwierc]

This looks much cleaner! Thx.

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