Make symbolic variables unpickle uniquely

[mwhansen]

Component: calculus

Issue created by migration from https://trac.sagemath.org/ticket/5466

[sagetrac-GeorgSWeber]

comment:1

Attachment: trac_5466.patch.gz

excellent work (tested with Sage 3.4.rc1)

[sagetrac-mabshoff]

comment:2

Merged in Sage 3.4.final.

Cheers,

Michael