Get pip update make targets running again

[maeve-fpf]

This cleans up the Makefile targets that update {dev-,}requirements.txt, and:

• runs them with a python 3.5 image to match current (legacy? :)) prod python version
• does not install all of the dev requirements just to be able to run pip-compile in the container

Without this, for reasons I don't entirely understand, pip install installs from dev-requirements.txt fine, but attempting to run any pip-compile step fails; logs from running manually in a container here: https://gist.github.com/maeve-fpf/a55248dcbf8420b0542c114f92beca1a

I suspect it may be related to the fact that if you pulled an official python docker image a long time ago, it'd have pip 19, but if you pull one now, it's on pip 20.

[maeve-fpf]

Testing note: please make sure you've pulled any python docker images that you have locally. make compile-pip-dependencies should no longer crash.

[maeve-fpf]

Updated this to take pip-tools out of dev-requirements.in, ran a hash update, fixed another probable typo.

My thinking is that since we're not doing anything to keep python:3.5-slim at a particular hash, it's pip version will continue to be upgraded, and if we pin pip-tools to a particular version, we could get in the same situation again where they are not compatible. Is that reasonable?

[chigby]

That makes a lot of sense to me. Thanks for recompiling the requirements (and fixing that other error), I'm going to merge this, now.