Various modules under taskcluster are doing ad-hoc url formatting or
requests to taskcluster services. While we could use the taskcluster
client python module, it's kind of overkill for the simple requests done
here. So instead of vendoring that module, create a smaller one with
a limited set of functions we need.
This changes the behavior of the get_artifact function to return a
file-like object when the file is neither a json nor a yaml, but that
branch was never used (and was actually returning an unassigned
variable, so it was broken anyways).
At the same time, make the function that does HTTP requests more
error-resistant, using urllib3's Retry with a backoff factor.
Also add a function that retrieves the list of artifacts, that while
currently unused, will be used by `mach artifact` shortly.
Instead of every file trying to get the top source directory having an
ad-hoc definition that gets wrong if the files gets moved around for
some reason, define it in a more central location.
