Minor changes in Python module

[kavitharaju]

This PR includes

• The argument filt is changed to filters. Using plural because filter is a python reserved keyword and also our variable is a list.
• Removed print() from code that was added during development
• In the JSON output, changed keys (#193 )
  • chapterNumber to c
  • verseNumber to v
• Handle failures in to_usx() (#183 )
  • Not raise error when unknown nodes are seen, allowing some valid USX output even when there are errors in parsing
  • Give better error report in case of failure
• Handle default attribute usage in \xt (#184 )
• Use multiple CPUs to run pytest, allowing much increase in performance. details here . This requires an additional plugin though