[devscripts/make_issue_template] Fix NameError under python3

devscripts/make_issue_template.py

@@ -16,6 +16,8 @@ def main():
     with io.open(infile, encoding='utf-8') as inf:
         issue_template_tmpl = inf.read()
 
+    __version__ = None
+
     # Get the version from youtube_dl/version.py without importing the package
     exec(compile(open('youtube_dl/version.py').read(),
                  'youtube_dl/version.py', 'exec'))