Fix the testsuite on Python3.8

There is a bug in Python3.8 (https://bugs.python.org/issue38688)
triggering an infinite recursion when copying a tree
in a subfolder of the current one. We're working around it
by using a list instead of an iterator, so that Python
won't "discover" the target folder as part of the source files.

This should fix #130
10 jobs for master in 2 minutes and 11 seconds (queued for 1 second)
Status Job ID Name Coverage
  Linting
passed #116835
linting:bandit

00:00:15

passed #116836
linting:codespell

00:00:16

passed #116839
linting:mypy

00:00:17

passed #116838
linting:pyflakes

00:00:15

passed #116837
linting:pylint

00:00:24

 
  Test
passed #116840
tests:archlinux

00:01:13

passed #116841
tests:debian

00:01:12

passed #116842
tests:debian_with_bubblewrap

00:01:45

100.0%
passed #116843
tests:fedora

00:01:12

passed #116844
tests:gentoo

00:01:43