Patch the Sage backend to work around issue in the SageMath package.

debian/changelog

+cantor (4:20.12.1-4) UNRELEASED; urgency=medium
+
+  [ John Scott ]
+  * Patch the Sage backend to work around issue in the SageMath package.
+    (Addresses: #972346)
+
+ -- Debian Qt/KDE Maintainers <debian-qt-kde@lists.debian.org>  Sun, 17 Jan 2021 18:34:54 -0500
+
 cantor (4:20.12.1-3) unstable; urgency=medium
 
   * Revert changes of previous version, not buildable.

debian/patches/series

 discount-build-verbose.diff
+work-around-Sage-no-root.diff

debian/patches/work-around-Sage-no-root.diff

+Description: work around unset SAGE_ROOT in Debian Sage package
+ In this shell script, '$1' refers to the Sage binary that is to
+ be used. The script is needlessly complex to accommodate for old
+ versions of Sage, and perhaps the author is unaware of Sage's
+ new command-line arguments.
+ .
+ Debian's `sage --root` is an empty string, so Cantor fails to
+ start it. This patch is more succinct by starting the Sage shell
+ which sets all variables appropriately, then starting
+ 'sage-ipython' as desired. This way, as opposed to having
+ '#!/usr/bin/sage --sh' as the shebang, still permits usage of
+ local Sage copies.
+Author: John Scott <jscott@posteo.net>
+Bug-Debian: https://bugs.debian.org/972346
+Forwarded: no
+Last-Update: 2021-01-17
+
+--- cantor-20.12.1.orig/src/backends/sage/cantor-execsage
++++ cantor-20.12.1/src/backends/sage/cantor-execsage
+@@ -1,23 +1,2 @@
+ #!/usr/bin/env bash
+-
+-export SAGE_ROOT="$($1 --root)"
+-
+-# Sage 9.2 stop source sage-env-config automatically, so do it manually
+-if [[ -f $SAGE_ROOT"/bin/sage-env" ]]; then
+-    if [[ -f $SAGE_ROOT"/bin/sage-env-config" ]]; then
+-        source "$SAGE_ROOT/bin/sage-env-config" >&2
+-    fi
+-    . "$SAGE_ROOT/bin/sage-env" >&2
+-elif [[ -f $SAGE_ROOT"/local/bin/sage-env" ]]; then
+-    if [[ -f $SAGE_ROOT"/local/bin/sage-env-config" ]]; then
+-        source "$SAGE_ROOT/local/bin/sage-env-config" >&2
+-    fi
+-    . "$SAGE_ROOT/local/bin/sage-env" >&2
+-elif [[ -f $SAGE_ROOT"/src/bin/sage-env" ]]; then
+-    if [[ -f $SAGE_ROOT"/src/bin/sage-env-config" ]]; then
+-        source "$SAGE_ROOT/src/bin/sage-env-config" >&2
+-    fi
+-    . "$SAGE_ROOT/src/bin/sage-env" >&2
+-fi
+-
+-exec sage-ipython -i --simple-prompt
++{ echo "sage-ipython -i --simple-prompt"; cat; } | $1 --sh