fix couchup for python3

[klaemo]

A tale of strings, bytes and pyhon3, I guess. This is probably the first line of python I've written in my life. Hooray! 😀 I consulted this fun document to understand what's going on.

Overview

couchup crashed in python3, because base64.b64encode expects a bytes-like object instead of a string. With this PR the string passed to base64.b64encode is properly encoded and decoded.

Testing recommendations

Run couchup with credentials with python3:
couchup replicate -a --login admin --password <my-password>

I was able to run couchup successfully on my DB with the change applied.

Related Issues or Pull Requests

fixes #1899

Checklist

• Code is written and works correctly;
• Changes are covered by tests;
• Documentation reflects the changes;

[wohali]

@klaemo Unfortunately I can't merge this because our style checker is failing. If you run sudo pip3 install black then make python-black-update and update your PR, it should solve the problem.

Thanks again for your PR!

[klaemo]

@wohali Done! ✅ Sorry I missed this step.