Use ruamel.yaml instead of pyyaml to avoid the "NO"rway problem

[rtaycher]

Describe the bug
openapi-python-client uses pyyaml which is sadly stuck on yamp 1.1 (yaml 1.2 came out in 2009, since then there's only been minor revisions). One of the main benefits of yaml 1.2 is that it treats strings like no/No/NO/OFF/Off/off as strings and not as booleans.

I ran into this issue and had the person maintaining the API ask what version of yaml I was using since it doesn't appear in

Ruamel also support comments if we ever say want to paste those in the documentation strings or something.

[dbanty]

I think I tried ruamel at one point and had some issues. Can’t remember what they were…

Definitely open to a PR!