sct_check_dependencies: use psutil instead of subprocess (#2960)
sct_check_dependencies: use psutil instead of subprocess (#2960)

* sct_check_dependencies: fetch system info (ram, cores) more cleanly #2799

* Remove unused imports.

* Properly report how cores are used #2849.
neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox