py3: remove dead code to make file descriptors non-inheritable
On Python 3, file descriptors are already non-inheritable by default.
/assign_reviewer @mercurial.review
Welcome to the Mercurial Merge Request creation process:
* Set a simple title for your MR,
* All important information should be contained in your changesets' content or description,
* You can add some workflow-relevant information here (eg: when this depends on another MR),
* If your changes are not ready for review yet, click `Start the title with Draft:` under the title.
More details here:
* https://www.mercurial-scm.org/wiki/ContributingChanges
* https://www.mercurial-scm.org/wiki/Heptapod