dummyssh: make sure we don't inherit files descriptor to the children This is one of the difference between python2 and python3 and could have been a reason why test hang with python2 + chg. This does not seems to help the hanging issue at all… However, now that this is written lets reduce the difference between python2 and python3. Differential Revision: https://phab.mercurial-scm.org/D11225