fix: Add google-auth to aiohttp extra

[clundin25]

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Fixes #384

[clundin25]

@parthea an extra sounds best to me.

[cojenco]

+1 to adding an extra instead as the async support is experimental and subject to change.

Additionally, we're also planning to integrate this library into python-storage in the near future. I'm trying to understand how the namespace packaging changes would impact this if any; cc @andrewsg

[andrewsg]

I would prefer extra, or else removing async testing. Async support is currently theoretical and not documented or supported.

[clundin25]

Out of curiosity, is there anyone working on stabilizing the Async API?

[andrewsg]

We don't have it on our roadmap at present as it would require a complete rewrite of python-storage, as well as an additional rewrite of this library in order to avoid having two separate versions of the library that need to be maintained simultaneously. This doesn't mean that async won't happen at all, just that it's not scheduled as of yet.

[clundin25]

Okay awesome. Are you a good point of contact for this? I have some questions I want to take offline.

[andrewsg]

Yep, feel free to reach out.

[clundin25]

I will fix the error tomorrow. Looks like some of the doc tests also use google auth