Add sage-train-track as an optional package

[videlec]

Part of #31164.

Upstream code at https://github.com/coulbois/sage-train-track.

Related:

• train-tracks #20154: train-tracks

CC: @sagetrac-coulbois @slel

Component: packages: optional

Keywords: train-tracks

Issue created by migration from https://trac.sagemath.org/ticket/31223

[mkoeppe]

comment:1

Sage development has entered the release candidate phase for 9.3. Setting a new milestone for this ticket based on a cursory review of ticket status, priority, and last modification date.

[slel]

Changed keywords from none to train-tracks

[slel]

Description changed:

--- 
+++ 
@@ -1 +1,7 @@
-See #31164. Upstream code at https://github.com/coulbois/sage-train-track.
+Part of #31164.
+
+Upstream code at https://github.com/coulbois/sage-train-track.
+
+Related:
+
+- #20154: train-tracks