requirements: add six to list of requirements

[samuelsimko]

The six python library is used in the LensIt code, but does not figure in the list of requirements.

LensIt is expecting mpi4py to be installed as well for better performance, which requires the libopenmpi-dev package to be installed on Debian-based Linux distributions, and the openmpi-devel package to be installed on RPM-based Linux distributions.
Maybe this can be added to the README, or to the requirements.txt as well ?

[Sebastian-Belkner]

Looks ok to me, but it seems the function using six has a bug anyway? (buffer is not defined).

For MPI, not sure if HPC compatibility on local devices is really needed.

[samuelsimko]

buffer is not defined for python3, but in python2 buffer creates a new buffer object. Since the function it is only called if python2 is used, I don't think the function using six is bugged.

As for MPI, you might be right about HPC compatibility not being needed on local devices.
I did get a bug on Yggdrasil because mpi4py was not installed in my virtual environnement though.

[Sebastian-Belkner]

I didn't know about the python2 behaviour, thank you.

You are definitely right, mpi4py should be added to the requirements / README at some point..