Add timeouts to http requests

[glumia]

Hi! First and foremost, thanks for this utility :)

This patch should solve the issue of occasional slowdowns on systems that use IPv6. You can find more details about it in the commit message.

I put the new constants above the from aws_google_auth ... imports in __init__.py and not below (as would be more common) as there's currently a 'latent issue' of circular imports that manifests itself if we do otherways. It could be solved by moving all the CLI-related code to a separate module and leaving only the constant declarations in the init.

You can find an example in https://github.com/glumia/aws-google-auth/tree/fix-circular-imports. I avoided adding those changes to this pull request both because it would be out of scope and because I don't know if such an invasive change to package's structure is welcome.