Antti, while I see some convenience in making %z parsing promiscuous, there is clear utility in adding %:z to strftime.  If we do that, not allowing the same for parsing will be odd.  Let's start with that.  A case for a promiscuous %z can be made later.

On a separate issue, note that datetime.isoformat() has recently grown a timespec option.  See issue 19475.  I wonder if it would make sense to add a tzspec option to control the way timezone is formatted.