util: add backport for shutil.which()

libagent/util.py

@@ -4,7 +4,6 @@ import contextlib
 import functools
 import io
 import logging
-import shutil
 import struct
 
 log = logging.getLogger(__name__)
@@ -219,7 +218,13 @@ def memoize(func):
 @memoize
 def which(cmd):
     """Return full path to specified command, or raise OSError if missing."""
-    full_path = shutil.which(cmd)
+    try:
+        # For Python 3
+        from shutil import which as _which
+    except ImportError:
+        # For Python 2
+        from backports.shutil_which import which as _which  # pylint: disable=relative-import
+    full_path = _which(cmd)
     if full_path is None:
         raise OSError('Cannot find {!r} in $PATH'.format(cmd))
     log.debug('which %r => %r', cmd, full_path)

setup.py

@@ -15,6 +15,7 @@ setup(
         'libagent.ssh'
     ],
     install_requires=[
+        'backports.shutil_which>=3.5.1',
         'ecdsa>=0.13',
         'ed25519>=1.4',
         'pymsgbox>=1.0.6',