Upgrade attrs version

[mahenzon]

There’s a backward-incompatible change in attrs 19.2.0 (2019-10-01): Removed deprecated Attribute attribute convert per scheduled removal on 2019/1.
https://www.attrs.org/en/stable/changelog.html

Now 18.1.0 is used

So we need to upgrade version and all dependent code that uses convert keyword
Tests may require an upgrade too (needs investigation)
More tests are welcome

[mahenzon]

Released in 1.4.0