py3: drop subprocess32 backport.

requirements.txt

@@ -20,7 +20,6 b' pygit2==0.28.2'
 repoze.lru==0.7
 redis==3.4.1
 simplejson==3.16.0
-subprocess32==3.5.4
 subvertpy==0.10.1
 
 six==1.11.0

vcsserver/subprocessio.py

@@ -24,7 +24,7 b' If not, see <http://www.gnu.org/licenses'
 """
 import os
 import logging
-import subprocess32 as subprocess
+import subprocess
 from collections import deque
 from threading import Event, Thread