Fix Python 3.8 SyntaxWarnings

[jhermann]

Annoying and easy to fix:

json_logging-1.0.5-py2.py3-none-any.whl/json_logging/util.py:187: 
SyntaxWarning: "is not" with a literal. Did you mean "!="?
  if key is not 'request' and key is not 'req':

[bobbui]

PR welcome if you want to do it, after that i can do a release