Downgrade request_store

[professor]

The 1.6.0 release is getting reverted. See https://github.com/steveklabnik/request_store/pull/99

Thank you for your contribution!

Check the following boxes:

• [ ] Wrote good commit messages.
• [ ] Feature branch is up-to-date with master (if not - rebase it).
• [ ] Squashed related commits together.
• [ ] Added tests.
• [ ] Added an entry to the Changelog if the new code introduces user-observable changes.
• [ ] The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences.