py3: updates to sage.interfaces.qsieve

[embray]

This has a few small changes, most of which were difficult to separate from each other:

• Use subprocess.Popen instead of os.popen
• Don't set os.environ['TMPDIR'] globally--by using subprocess.Popen we
  can update the environment just for the subprocess
• str/bytes fixes for Python 3

Previously this was also reusing the same tmpdir for all qsieve calls, but there does not appear to be any actual reason to do that since it appears to clean up all the files in its temp dir between runs anyways.

Component: python3

Keywords: flintqs

Author: Erik Bray

Branch/Commit: 400eabc

Reviewer: Jeroen Demeyer

Issue created by migration from https://trac.sagemath.org/ticket/24789

[jdemeyer]

comment:2

Makes sense. positive review if tests pass.

[jdemeyer]

Reviewer: Jeroen Demeyer

[fchapoton]

comment:4

ok

[vbraun]

Changed branch from u/embray/python3/sage-interfaces-qsieve to 400eabc