Don't wait for unrequired status checks.

[Mariatta]

In backport PRs, if all the required status checks have completed, then go ahead and merge the PR.
Don't need to wait for the unrequired status checks.

[Mariatta]

GitMate.io thinks possibly related issues are python/core-workflow#72 (Add a status check for reverted commits), python/core-workflow#76 (Turn off code coverage status checks), python/core-workflow#19 (Don't wait for allowed_failures on travis builds?), #109 ("Backport status check is done, and it's a success white_check_mark ." hiccup), and python/core-workflow#269 (Improve guidance to core devs on checking CLA status).

[Mariatta]

Required status checks for a branch can be looked up via: https://developer.github.com/v3/repos/branches/#get-required-status-checks-of-protected-branch

[Mariatta]

The actual API we want to use is: https://developer.github.com/v3/repos/branches/#list-required-status-checks-contexts-of-protected-branch

But this is currently blocked because miss-islington doesn't have the right permission to access that API endpoint.

[arhadthedev]

No longer relevant after python/core-workflow#498.