Add google-auth-httplib2 as dependency

[honnix]

Description

Adding a new dependency.

Motivation and Context

As discussed here #2361 (comment)

This is used by https://github.com/google/google-api-python-client/blob/v1.6.6/googleapiclient/discovery.py#L371

Another option is to fix luigi.contrib.bigquery somehow.

Have you tested this? If so, how?

Manually installing this package worked for me.

[ulzha]

tox.ini is just adding it as a dependency for tests; didn't you mean to add it as a dependency for the real thing?

It's a bit unfortunate that contrib modules aren't separate packages with their own setup.pys; that would be the place to declare dependencies...

[honnix]

I did mean to put it as real thing. Sorry for the mistake. Another PR coming.

[honnix]

Now the problem is, I don't know where to put it. setup.py doesn't seem to be a good place. @ulzha as you mentioned, how do we handle contrib deps?

[Tarrasch]

As you maybe have noticed by now, we only list the dependencies used in "core" luigi in steup.py here. It's not the best solution, but it have worked so far. I would be really glad if we did manage to create packages for the various contrib packages, at least the larger ones. I tried once but never really got started on it on #2170.

[honnix]

That looks like a big change in code structure. Not so sure we want to spend that much effort atm.