Make fetch compatible with py 3.5

Running `fetch v8` with Python 3.5 produces the following error:
TypeError: the JSON object must be str, not 'bytes'

Adding `.decode("utf-8")` makes it compatible with both versions.

Change-Id: Ib0699b61b24f191559c30f1e7ca8d2c919803d03
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2154108
Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>

fetch.py

@@ -267,14 +267,14 @@ def run_config_fetch(config, props, aliased=False):
   cmd = [sys.executable, config_path + '.py', 'fetch'] + props
   result = subprocess.Popen(cmd, stdout=subprocess.PIPE).communicate()[0]
 
-  spec = json.loads(result)
+  spec = json.loads(result.decode("utf-8"))
   if 'alias' in spec:
     assert not aliased
     return run_config_fetch(
         spec['alias']['config'], spec['alias']['props'] + props, aliased=True)
   cmd = [sys.executable, config_path + '.py', 'root']
   result = subprocess.Popen(cmd, stdout=subprocess.PIPE).communicate()[0]
-  root = json.loads(result)
+  root = json.loads(result.decode("utf-8"))
   return spec, root