Lower six version requirements

[soltanmm]

cc @jtattermusch
Fixes #5247.

[jtattermusch]

@soltanmm , can you please incorporate changes from https://github.com/grpc/grpc/pull/5176/files (after verifying they are not breaking anything)?

[nathanielmanistaatgoogle]

Looks good; merge at your leisure.

[soltanmm]

Huh. It turns out that because pip eagerly grabs higher version numbers on our dependencies (like sphinx, enum34, and futures), our transitive dependencies almost always end up requiring six>=1.9, and will almost always end up vomiting strange version conflict error messages to our users. Furthermore, the only way we'd get any benefit out of a PR like this one is if we decrease the required version to 1.8 or below (for various Ubuntu flavors).

I don't think this is a good idea anymore.