Trac #34935: Followup to #34547: fix emacs sage-shell-mode

It turns out that sage-shell-mode in emacs uses the variable
`interfaces`, deleted from `sage.interfaces.all` in #34547. We restore
that variable (which is not used anywhere in the Sage library) along
with a comment.

URL: https://trac.sagemath.org/34935
Reported by: jhpalmieri
Ticket author(s): John Palmieri
Reviewer(s): Matthias Koeppe

build/pkgs/configure/checksums.ini

@@ -1,4 +1,4 @@
 tarball=configure-VERSION.tar.gz
-sha1=cc57d00f6f8fd1c5123b6f7dee07d8c6c717859d
-md5=62d1ead9dd1ddfa11a1c4164f197bddb
-cksum=580469152
+sha1=76789b1961447494e1413ee2fd1faea93c78fdcc
+md5=1835938ed1e66b7225343ca0b551036d
+cksum=3073282865

build/pkgs/configure/package-version.txt

@@ -1 +1 @@
-0f25b00a8e24881db418b69dcfe499d6437aa5b7
+4c99a3fb2975d998daa5593c603b9d58dc9a2902

src/sage/interfaces/all.py

@@ -43,3 +43,9 @@
 lazy_import('sage.interfaces.read_data', 'read_data')
 lazy_import('sage.interfaces.scilab', 'scilab')
 lazy_import('sage.interfaces.tachyon', 'tachyon_rt')
+
+# The following variable is used by sage-shell-mode in emacs:
+interfaces = ['gap', 'gap3', 'giac', 'gp', 'mathematica', 'gnuplot',
+              'kash', 'magma', 'macaulay2', 'maple', 'maxima',
+              'mathematica', 'mwrank', 'octave', 'r', 'singular',
+              'sage0', 'sage']