Use PR username when no CircleCI project

[GeorgianaElena]

1. When implementing Update tests #511, I didn't realize that when a TLJH fork doesn't have CircleCI setup there's no CIRCLE_PROJECT_USERNAME env var. Instead there is a CIRCLE_PR_USERNAME that can be used.

Example:

• My TLJH fork has CircleCI setup, there are these env vars available:
  
• @minrk and @jtpio recent PRs don't have these env vars and instead have:
  
  

2. Another way to solve this issue is installing from git refs (available since pip 10). However this would imply some gymnastics parsing the CIRCLE_PULL_REQUEST var.
   
   like this:

index = CIRCLE_PULL_REQUEST.find("/pull/")
BOOTSTRAP_PIP_SPEC="git+" + CIRCLE_PULL_REQUEST[:index] + ".git@ref" + CIRCLE_PULL_REQUEST[index:] + "/head"

So that in the end to have BOOTSTRAP_PIP_SPEC be:
git+https://github.com/jupyterhub/the-littlest-jupyterhub.git@ref/pull/527/head

What do folks think is the better way?

[jtpio]

Thanks @GeorgianaElena for looking into this 👍

No strong opinion here, but it looks like the current change (option 1) works well and is quite explicit?

[betatim]

Looks good to me, so merged it.

Do you know why the codecov checck was missing/pending?