Bug 1611014 - Make sure Taskcluster root_dir is a unicode string. r=rstewart

This fixes an issue when running "mach taskgraph" with the --root parameter
from the commandline. We use this regularly when working on Thunderbird task
configuration.

The initial list of kind names is read with os.listdir(), which returns
non-unicode strings when given a non-unicode string. As a result, some tasks
fail to validate because there's a non-unicode string where a unicode string
is expected.

Differential Revision: https://phabricator.services.mozilla.com/D60775

--HG--
extra : moz-landing-system : lando

taskcluster/taskgraph/generator.py

@@ -7,7 +7,7 @@ import logging
 import os
 import copy
 import attr
-from six import text_type
+from six import text_type, ensure_text
 
 from . import filter_tasks
 from .graph import Graph
@@ -115,7 +115,7 @@ class TaskGraphGenerator(object):
         """
         if root_dir is None:
             root_dir = 'taskcluster/ci'
-        self.root_dir = root_dir
+        self.root_dir = ensure_text(root_dir)
         self._parameters = parameters
         self._target_kind = target_kind