Python3 build

[paskino]

Updates scripts and packages to build with SIRF with python3.

[paskino]

Notice that it is needed to install cython and cython3 because some of the prerequisites in the SuperBuild build with python2 apparently.

[paskino]

This also means that some packages in the SuperBuild pick the wrong python version and I'm not entirely sure which one.

[KrisThielemans]

I think this will be premature to merge for 2.2. We can of course always release another VM afterwards.

Possibly this could be resolved by tackling SyneRBI/SIRF-SuperBuild#48.

[rijobro]

As discussed previously, we could potentially switch to 20.04 meaning that it would use python3 by default. Might fix some problems without any effort on our part.

Would require fixing SyneRBI/SIRF-SuperBuild#413.

[KrisThielemans]

without any effort on our part

seems somewhat wishful thinking, given Alex's struggles 😉

In any case, finding correct Python versions will remain a bane. Maybe not on a clean Ubuntu 20.04 VM, but as soon as conda gets involved, it'll be a load of fun to be sure we don't mix conda and system python.

[KrisThielemans]

looks good to me. aside from these 2, ready to merge

[KrisThielemans]

Two comment that remain to be resolved. In addition, https://github.com/SyneRBI/SyneRBI_VM/blob/master/documentation/Getting-started-with-SIRF.md needs updating as it says python and talks about spyder versions.