Add missing distros/conda.txt for primecountpy.

Test run: https://github.com/sagemath/sagetrac-mirror/runs/5173070066?check_suite_focus=true

CC: @isuruf @dimpase @saraedum @mkoeppe

Component: build

Author: Tobias Diez

Branch/Commit: 613198a

Reviewer: Dima Pasechnik

Issue created by migration from https://trac.sagemath.org/ticket/33330