Remove formencode dependency

[amol-]

Formencode no longer supports Python3.9 and seems to have been unmaintained for a few years.
We should remove the need for it from Ming.

[brondsem]

Looks like it has been maintained again in recent years. Latest release supports 3.10 http://www.formencode.org/en/latest/whatsnew-2.0.html

There's even a commit adding 3.11 supports. AFAICT it hasn't been released yet, but also is just build changes, no code changes. formencode/formencode@25749f8

In Ming, formencode is already an optional dependency, but still could be replaced potentially.