The `tenacity` dependency costs a lot and brings a little

[exarkun]

We use tenacity in just one place, test_iputil.py, for a function that we should probably mostly throw away anyway.

The functionality we use from tenacity can be re-implemented in 19 lines (not counting docs / whitespace).

Tenacity is not very big but it has packaging complications on PyPy? and it's easier to re-implement its functionality than fix the packaging issues.

Let's drop the dependency.

[GitHub <noreply@…>]

In 2dfabf79/trunk:

Merge pull request #1270 from exarkun/3989.drop-tenacity-dep

Fixes: ticket:3989