cubicweb-mercurial-server #4542773 use python-hglib [validation pending]

instead of mercurial directly

prioritynormal
typetask
done in0.6.0
load1.000
load left0.000
closed by#904997599077 Use hglib and upgrade tests to cw 3.19 API (closes #4542773)
patchUse hglib and upgrade tests to cw 3.19 API (closes #4542773) [applied]