Follow up on #29922.
The doctester uses RealIntervalField for doctests involving numerical tolerances. This is an obstacle to doctesting modularized subset distributions because RealIntervalField depends on a compiled library (MPFI).

We should eliminate this by rewriting it in a more elementary way.

As of this ticket, we just issue a warning if RealIntervalField is not available.

CC: @kliem @sagetrac-tmonteil

Component: doctest framework

Author: Matthias Koeppe

Branch/Commit: db154ec

Reviewer: Travis Scrimshaw

Issue created by migration from https://trac.sagemath.org/ticket/32585