As mentioned in the issue here: pallets-eco#147
The PyJWT dependency used in current version of FlaskJWT seems to be using an older version of PyJWT that is causing following error:
ImportError: cannot import name 'Mapping' from 'collections' 

Upgraded the PyJWT version to use version 2.6.0 to resolve this issue

Updated PyJWT version to 2.6.0

Python 3 doesn't support str.decode method. 
So, trying to decode access_token in python 3 raises following error:
AttributeError: 'str' object has no attribute 'decode'

Added try except block to prevent that.