Make RAIDLevel an abstract class using abc.
Make RAIDLevel an abstract class using abc.

RAIDLevel was, already, an abstract class, but using abc makes this more
obvious and harder to break.

Signed-off-by: mulhern <7b51bcf507bcd7afb72bf8663752c0ddbeb517f6@redhat.com>

vpodzime/blivet,rvykydal/blivet,vpodzime/blivet,AdamWill/blivet,vojtechtrefny/blivet,rhinstaller/blivet,AdamWill/blivet,jkonecny12/blivet,rvykydal/blivet,jkonecny12/blivet,dwlehman/blivet,vojtechtrefny/blivet,dwlehman/blivet,rhinstaller/blivet