tox under fakeroot: fix finding setup_docs, fixes #7391

tox.ini

@@ -10,6 +10,7 @@ requires =
 	wheel
 	setuptools_scm
 [testenv]
+package = editable  # without this it does not find setup_docs when running under fakeroot
 setenv =
    fuse2: BORG_FUSE_IMPL=llfuse
    fuse3: BORG_FUSE_IMPL=pyfuse3