Add import sys
to your main.py and add these lines.
# inject './lib' dir in the path so that we can simply do "import ndb" or whatever there's in the app lib dir.
if 'lib' not in sys.path:
sys.path[0:0] = ['lib']
Create the lib folder in the root of your project and copy the unirest folder in lib folder.
You can now import unirest and use your library.
But, why not use the standard method and library provide by Google App Engine ?