infra: accelerate builds

[codeboten]

Some build time improvements:

• split lint/docker-tests/docs into their own steps. Since lint is usually the thing that fails anyways, it's good to have it run first. We could make the build depend on this step to prevent slowing other builds waiting in the pipeline (since we only have 5 workers)
• move all pip install commands into a single line per test environment. this reduces the overhead of calling the pip command separately multiple times per environment.
• removed pip upgrade command. This one I'm not sure about, it could potentially lead to broken builds in the future, but its taking roughly 10s each time it's done, which adds up as we add more extensions.

[toumorokoshi]

Cool! I'd probably say the overhead of running pip multiple times is not a big issue (Python vm startup ~100ms, maybe an additional 100ms to load the core bytecode). But the lint re-order makes a lot of sense! Nice stuff.

[toumorokoshi]

approving assuming build will pass. Thanks!

[lzchen]

Any reason why we still need this line if it is run in travis.yml?

[codeboten]

Unfortunately, upgrading pip in travis before bringing up virtualenvs via tox was part of a failed experiment. I've reverted the change in the travis file, and kept the pip install -U command for the versions of python that needed it.

[lzchen]

LGTM