[python][rfc] Change gdb.Block to use attributes instead of getter methods.

[Tom Tromey]

Thiago> But now that I think of it, I'm not sure why I added the method in the
Thiago> first place. The iterator can be used without an explicit call to this
Thiago> method, by directly using gdb.Block in a for or in statement. Or even by
Thiago> explicitly calling gdb.Block.__iter__. I'm inclined to remove
Thiago> "itersymbols", what do you think?

That sounds good to me.

Tom