Remove unnecessary submodules

[bmyerz]

@senderista and I discussed that

• six and dateutil may be remnants of fixing the requests dependency

[senderista]

We should remove requests as well, in fact all third-party submodules should be converted into proper install dependencies. I will have to revisit the requests dependency (I converted it to a submodule because I couldn't find a release version corresponding to the commit that fixed my SSL issue).

[jortiz16]

Currently, there's an issue with six -- that repository no longer exists. Seems to be ok if we replace with https://github.com/gsutil-mirrors/six for now but who knows how long that will be around for.

Any temporary fix for this would be great. I would help, but I don't know how to update submodules and I'm not sure what functionality we need from these dependencies.

[senderista]

d47ed0f temporarily fixed the six breakage by changing the remote to https://github.com/gsutil-mirrors/six.