Fix: Encode the output file properly for Python 2 runs

Avoid compatibility errors with the `open(...)` function in Python 2 for
setting the file's encoding.

Sometimes in case an old architecture and `cms-sw` release is being
used, a Python 2 interpreter is available for executing the
`run_the_matrix_pdmv.py` module and there are compatibility issues with
the encoding argument.

core/utils/run_the_matrix_pdmv.py

@@ -4,6 +4,7 @@
 from __future__ import print_function
 
 import argparse
+import codecs
 import importlib
 import inspect
 import json
@@ -312,7 +313,7 @@ def main():
     print('All workflows:')
     print(json.dumps(workflows, indent=2, sort_keys=True))
     if opt.output_file:
-        with open(opt.output_file, 'w', encoding='utf-8') as workflows_file:
+        with codecs.open(opt.output_file, 'w', encoding='utf-8') as workflows_file:
             json.dump(workflows, workflows_file)