Fix seq widgets

[krysros]

This pull request should fix functional tests of Pylons/deform#518.

[stevepiercy]

@krysros I started the run, and so far lint failed, but it's not clear to me why. Perhaps you can make sense of it?

https://github.com/Pylons/deformdemo/runs/6872633267?check_suite_focus=true

We'll see about the other builds. At this point, I am OK with dropping Python 3.6 and adding 3.10, too.

[krysros]

@stevepiercy

I have no idea about lint fail yet. I passed all functional tests running locally from deform with the changes applied in Pylons/deform#523, Pylons/deform#524, #123 and with the following modifications of https://github.com/Pylons/deform/blob/1a6e9e07a9a7fa0208c1aa219649d99ebc132eb7/run-selenium-tests.bash#L31

git clone --branch fix-seq-widgets https://github.com/krysros/deformdemo.git deformdemo_functional_tests