[MERGE] Allow parent_providers to not have a get_parents attribute.

[Aaron Bentley]

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Robert Collins wrote:
> This makes the deprecation of get_parents easier to work with by
> removing the need to have get_parents on ParentProvider's.

bb:resubmit

ParentProviders are a private API and we can freely remove anything we
like from them.  But Graph is not a private API, and it provides a
get_parents method.

You can't just set the method to None some of the time.  You can easily
implement it in terms of get_parent_map.  You can even copy & paste the
code from a ParentProvider.  You can deprecate it.

Or else, if for some reason, you think an API break is necessary, you
can remove the method, and document that API break.

But setting it to None some of the time doesn't make sense.

Aaron
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