better fix for mixing @parallel with numerical constant preparsing

[craigcitro]

The problem in #4312 got resolved along the way, we provide a doctest.

Original description:

As noted on #4312, @parallel and the new preprocessing of constants don't play together very well. The fix at #4312 provides a fix, but a better fix is needed.

Component: misc

Author: Jonathan Kliem

Branch/Commit: 9344a7c

Reviewer: Kwankyu Lee

Issue created by migration from https://trac.sagemath.org/ticket/4545

[kwankyu]

comment:5

Apparently the problem just went away.