Fix unpinned dependencies tests towards botocore and urllib3

[adferrand]

Currently unpinned dependencies tests are failing, see latest Travis nightly run: https://travis-ci.com/certbot/certbot/builds/112182519

This is because:

1. New version of requests has been released yesterday (2.22.0), and it relax its transitive dependency from urllib<1.25 to urllib<1.26
2. Current version of botocore in the route53 certbot dns plugin still requires urllib<1.25

Several issues have been raised in botocore project about this, with one issue tracking down the update of urllib3 dependency in botocore: boto/botocore#1733

So hopefully botocore will be update before our next release, but nonetheless I make a PR here to fix the situation until that, by capping our own transitive dependency for urllib3 to <1.25. I prefer doing that instead of capping requests version, because it still compatible with urllib3 1.24.x branch, and we should keep the benefit of the latest improvements in requests.

Once botocore is updated, this PR should be reverted.

[adferrand]

Relevant nightly build can be seen here: https://travis-ci.com/certbot/certbot/builds/112210832

[bmw]

Whoever ends up resolving pypa/pip#988 instantly becomes one of my favorite people. (@adferrand, try not to let this comment nerdsnipe you 😝 I think it's a big and fairly political task.)

What I think we should do here is this:

1. Pin back urllib3 in certbot-dns-route53/setup.py with a comment explaining that this is needed to make installation work with pip due to pip needs a dependency resolver pypa/pip#988, allow the use of urllib3 1.25.x boto/botocore#1733, and pip preferring the newest version of all dependencies. I'd also say that if allow the use of urllib3 1.25.x boto/botocore#1733 has been resolved, the dependency on urllib3 specified here can be safely removed. I think the fix should go in the route53 plugin since that is the only component affected and the comment is useful to describe what is going on to package maintainers who may need to fix this up on their end.
2. Create an issue in the current milestone for us to remove this pinning with a link to the boto3 botocore issue.

What do you think?

[adferrand]

I added #7083 to track revert on this PR.