* Add python-requires in setup.py

* Update setup.py

…rs#9011)

* Fix dist install test by ensuring that we use Python 3

Now that we have dropped Python 2 support, we need to make sure this
install test uses Python 3.

* Update Docker image to install Python 3 version of setuptools

* Run pip3 instead of pip

googletest uses a Python script in its build which is not compatible
with Python 3. Unfortunately we can't easily upgrade googletest right
now, so this commit works around the problem by putting a python symlink
pointing to Python 2 in the $PATH.

It appears that this extra conditional in travis-test.sh is no longer
necessary, and in fact we need to avoid using a custom version of
OpenSSL since that is causing its own error.

I am hoping this will fix the error we are getting as described here:
rvm/rvm#5014