Address pylint raise-missing-from warning

Versions 2.6.0 and later of pylint adhere to PEP 3134
and trigger a 'raise-missing-from' warning (W0707) when
chained exceptions are raised implicitly.

The 'from' keyword is a Python3.x feature, that is why
six.raise_from is used for Python2.x compatibility.

Kudos to @sechkova and @jku for showing how to do this
(theupdateframework/python-tuf#1116).

Signed-off-by: Lukas Puehringer <lukas.puehringer@nyu.edu>

in_toto/runlib.py

@@ -24,6 +24,7 @@
  - Return Metablock containing a Link object which can be can be signed
  and stored to disk
 """
+import six
 import glob
 import logging
 import os
@@ -212,8 +213,8 @@ def record_artifacts_as_dict(artifacts, exclude_patterns=None,
  os.chdir(base_path)
 
  except Exception as e:
- raise ValueError("Could not use '{}' as base path: '{}'".format(
- base_path, e))
+ six.raise_from(ValueError("Could not use '{}' as base path: '{}'".format(
+ base_path, e)), e)
 
  # Normalize passed paths
  norm_artifacts = []