ruamel sometimes saves with extra "-"

[vsoch]

Describe the bug

I don't know how to reproduce this (yet) but sometimes ruamel saves a file with an added "-" e.g.

on:
-  schedule:
+  schedule: -
    cron: "xxxxx"

And the only fix is to manually delete it. This is obviously a bug!

[vsoch]

I think it could be based on what it determines to be a commented (text) block vs. a dictionary, where the first has more than some expected number of spaces.

[vsoch]

okay I think I know what's happening - it's a list that it moves to the previous line, so instead of a list of dicts (correct) it is read as a text extended (with dict content). I had to do a fix here: singularityhub/singularity-hpc#610