add empty slots to _resultbase, whose subclasses have slots

[ariebovenberg]

Summary of changes

Closes #1186

Pull Request Checklist

• [x] ~~Changes have tests~~ n/a (see questions below)
• [x] Authors have been added to AUTHORS.md
• [x] News fragment added in changelog.d. See CONTRIBUTING.md for details

Questions

• regarding tests: probably not needed. I discovered the slots issue with slotscheck, a tool I wrote. Of course, If you like, I can add it to tox/CI as I've done for instagram/LibCST and dry-python/returns.

[ariebovenberg]

It took me a while to decypher: This test fails if you don't have the dateutil timezone file installed. Please read the README because there was no mention if running the updatezinfo.py there.

[ariebovenberg]

The remaining failures seem unrelated (pypy3 Error: PyPy 3.6 not found). If you like I can fix the instructions about running updatezinfo.py as well.