feat: Add a little bit of typing to google.api_core.retry

[asottile-sentry]

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[asottile-sentry]

this library has python_requires >= 3.7 so (1) unicode_literals does nothing and (2) this is the first version which supports __future__.annotations

it might be a good idea to run https://github.com/asottile/pyupgrade on this codebase to remove the remaining python 2isms

[parthea]

Thanks! I've captured this TODO in #524

[asottile-sentry]

👋 I've rebased this to resolve conflicts (again) -- could it get reviewed?

[parthea]

Thanks @asottile-sentry!

[parthea]

Thanks! I've captured this TODO in #524

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