Enable sage backend by default only if SAGE_ROOT is set
Enable sage backend by default only if SAGE_ROOT is set

It's possible to install the Sage system as a normal Python package, as
e.g. in Fedora/Debian. In this case, presence of `sage.all` is not an
indicator whether the user is running Sage, and checking for 'SAGE_ROOT'
is more reliable.

fredrik-johansson/mpmath,JensGrabner/mpmath,JensGrabner/mpmath