Switch from retrying to tenacity

[pradyunsg]

Looks like retrying is no longer maintained: rholder/retrying#79

We can switch to using https://github.com/jd/tenacity instead, which is "mostly" API compatible, and seems to be in a better state -- maintenance wise.

Originally posted by @chrahunt in #7363 (comment)

[yeraydiazdiaz]

For visibility, I had a go at this issue in #7592 but found there was some complexity due to tenacity depending on futures for Python 2.7.