Upgrade httpretty to 0.9.4 for Python 3 support

[cclauss]

Helps with #3309

Proposed fixes:

Upgrade development requirement httpretty from 0.8.3 to 0.9.4 for Python 3 support.

Features:

• includes tests covering changes
• includes updated documentation
• includes user-visible changes
• includes API changes
• includes bugfix for possible backport

Please [X] all the boxes above that apply

[amercader]

We've had lots of problems in the past with httpretty versions and tests that use Solr or the DataPusher, and sure enough there are already 2 failures related. It might just be that the tests need updating because of some change in httpretty, but in any case we need to get these fixed or rewritten before merging.

[cclauss]

The root cause is that Circle CI is testing on Python 2.7.6 (current is 2.7.15) and the current httpretty is relying on ssl.SSLContext which was added in Python 2.7.9.

[cclauss]

In CircleCI on the pip install -r dev-requirements.txt step we are getting these warnings:

• httpretty 0.8.3 has requirement urllib3==1.7.1, but you'll have urllib3 1.23 which is incompatible.
• mox3 0.25.0 has requirement pbr!=2.1.0,>=2.0.0, but you'll have pbr 1.10.0 which is incompatible.