I hope this saves somebody some time. Posting because I found very little concerning URLFetch error.
I was suddenly receiving "WARNING 2017-06-28 23:09:40,971 urlfetch_stub.py:550] Stripped prohibited headers from URLFetch request: ['Host']" on a working Google Places Application.
The update for Google Cloud SDK 161.0.0 was kind enough to inform me that my version of Python was out of date. Ubuntu 14.04 is frozen at Python v. 2.7.6
sudo apt-get install build-essential checkinstall
sudo apt-get install libreadline-gplv2-dev libncursesw5-dev libssl-dev libsqlite3-dev tk-dev libgdbm-dev libc6-dev libbz2-dev
sudo apt-get update
App engine is currently on Python 2.7.12, I tried compiling the source, but it would not install, had success with this package and looks like it is at the end of support.
sudo add-apt-repository ppa:fkrull/deadsnakes-python2.7
sudo apt-get update
sudo apt-get install python2.7
I had to repeat the update and install steps several times to get the package to install. (Apparently, dependencies have other dependencies.)
python -V
should return: Python 2.7.12