futures: switch to absolute and relative imports
This makes the package conform with our importing policy,
silencing a number of warnings. It also makes the package usable
when it isn't named "concurrent.futures."
Differential Revision: https://phab.mercurial-scm.org/D3264
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https://www.mercurial-scm.org/wiki/ContributingChanges
If you just want a checklist to follow, you can go straight to
https://www.mercurial-scm.org/wiki/ContributingChanges#Submission_checklist
If you can't run the entire testsuite for some reason (it can be
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any files you've modified and run `python contrib/check-commit` on any
commits you've made (for example, `python contrib/check-commit
273ce12ad8f1` will report some style violations on a very old commit).