Add Splunk 8 and Python 3 support

• Add support for Splunk 8 using Python 3. (earlier versions worked fine on Splunk 8, but used Python 2)
• Update python libraries:
  • jmespath 0.9.4 -> 0.10.0
  • splunk-sdk 1.6.6 -> 1.6.13
  • six 1.15.0 (new)
• Removed newlines from the jsonformat output=makeresults output. Whoops.
• Add Python 3 optimization for jsonformat order=preserve based on the fact that the dict data type now preserves order by default in Python 3.7 and later.
• Minor internal exception handling improvements
• Logging level is now WARNING by default (only impacts jsonformat) which should remove some noise in the internal logs. (More cleanup could be used here.)