Update cmake configuration for disabling builds against python 3

[amebel]

To use Python2 only from the build directory run the following
rm CMakeCache.txt && cmake -DCMAKE_DISABLE_FIND_PACKAGE_Python3Interp=TRUE ..

[ferrouswheel]

This forces people to use Python 3.4 because you've changed to explicitly calling nosetests-3.4 :-(

The cmake variable to disable Python 3 detection could also be better named (unless this is an inbuilt variable provided by CMake), and probably should be added to the build documentation.

[amebel]

Installing using pip3, nose executable is called nosetests-3.4 not nosetests3.

It is inbuilt.

Done https://wiki.opencog.org/w/Building_OpenCog#Building

[ferrouswheel]

If pip3 points to a pip installed with Python 3.4 then yeah, it will install nosetests-3.4.

Ubuntu 18.04 uses Python 3.6, so when you use pip3 to install nosetests you won't have nosetests-3.4. A lot of our SNet services rely on python 3.6 for async/await functionality, so if anyone is working on OpenCog and SNet, then this will probably cause issues for them.

Before I upgraded to 18.04 I had both Python3.4 and Python3.6 installed on my system. With the cmake config in this PR, it would find Python3.6 as the interpreter, but it would use nosetests from 3.4 - potentially introducing confusing errors.

I think the ideal solution would be to make sure nosetests matches the version of the python interpreter that was previously found.

[amebel]

I had tested against python 3.5.2 in ubuntu 16.04 container, and just tested against python 3.6.5 in ubuntu 18.04 container, & python 3.6.6 in conda, in all cases it is nosetests-3.4. Is there some option in pip that gets(or renames) nosetest-3.5/3.6?

[ferrouswheel]

Wow, I just checked on an Ubuntu 16.04 box and even though I'm installing nose 1.3.7 with python 3.5 it still calls it nosetests-3.4!

I guess we discovered a bug with nosetests distribution.

I found this issue nose-devs/nose#1009 but it's from 2016. I've updated it with why it's gone wrong (it's due to it being packaged as a whl)

[vsbogd]

I don't have nosetests-3.4 and have nosetests3 instead probably because I don't use pip, just installed python3-nose in Ubuntu 18.

[vsbogd]

I see that README.md doesn't describe how to install python to build Atomspace properly. I think it makes sense add this information to the documentation.