constrain ruamel.yaml version for now

From https://sourceforge.net/p/ruamel-yaml/code/ci/0.17.22/tree/CHANGES

> plain scalars: put single words longer than width on a line of their own,
> instead of after the previous line (issue 427, reported by Antoine Cotten).
> Caveat: this currently results in a space ending the previous line.

One of the side effects is that yamlfmt and yamllint become in conflict
when using ruamel.yaml 0.17.22.

Therefore constrain ruamel.yaml to be <= 0.17.21.

Resolves #47

Related: #49

Maybe related: #48

setup.py

@@ -9,7 +9,7 @@
     version='0.0.0',
 
     install_requires=[
-        'ruamel.yaml>=0.16.10',
+        'ruamel.yaml >=0.16.10, <=0.17.21',
     ],
 
     scripts=[