slapos-sr-testing: Change ptyprocess version to whl

ptyprocess=0.6.0 uses flit as build backend
5 jobs for dev/buildout-3-prepare-for-mr in 0 seconds
Status Job ID Name Coverage
  External
failed ERP5.UnitTest-Master-Buildout3

03:10:49

passed ERP5.UnitTest-Master-Buildout3

02:13:15

failed ERP5.UnitTest-Master-Buildout3

01:55:17

failed ERP5.UnitTest-Master-Buildout3

02:16:16

failed ERP5.UnitTest-Master-Buildout3

01:38:49