sys-apps/portage: Add threads(+) to PYTHON_REQ_USE

This should help prevent erors like the following:

  File "/usr/lib64/python2.7/site-packages/portage/locks.py", line 11, in <module>
    import multiprocessing
ImportError: No module named multiprocessing

Package-Manager: portage-2.3.2

sys-apps/portage/portage-2.3.2.ebuild

@@ -9,7 +9,7 @@ PYTHON_COMPAT=(
  python3_3 python3_4 python3_5
  python2_7
 )
-PYTHON_REQ_USE='bzip2(+)'
+PYTHON_REQ_USE='bzip2(+),threads(+)'
 
 inherit distutils-r1 multilib