tests: Only use subprocess32 package on Python 2.x

This is how the package documentation recommends and it's less likely to
interfere with new features in stdlib subprocess module.

.ci/ci_lib.py

@@ -10,7 +10,10 @@ import shutil
 import sys
 import tempfile
 
-import subprocess32 as subprocess
+if sys.version_info < (3, 0):
+    import subprocess32 as subprocess
+else:
+    import subprocess
 
 try:
     import urlparse

tests/requirements.txt

@@ -8,7 +8,7 @@ cffi==1.14.3  # Random pin to try and fix pyparser==2.18 not having effect
 pycparser==2.18  # Last version supporting 2.6.
 pytest-catchlog==1.2.2
 pytest==3.1.2
-subprocess32==3.5.4
+subprocess32==3.5.4; python_version < '3.0'
 timeoutcontext==1.2.0
 # Fix InsecurePlatformWarning while creating py26 tox environment
 # https://urllib3.readthedocs.io/en/latest/advanced-usage.html#ssl-warnings

tests/testlib.py

@@ -18,7 +18,10 @@ except ImportError:
     import ConfigParser as configparser
 
 import psutil
-import subprocess32 as subprocess
+if sys.version_info < (3, 0):
+    import subprocess32 as subprocess
+else:
+    import subprocess
 
 import mitogen.core
 import mitogen.fork