Move ConverterBase to the converters.base?

[IAlibay]

Just having a look at things, and I realise that ConverterBase essentially is just a small subclass of IOBase, but I wonder if it might make more sense to have that in converters.base so that it's more logically situated?

[orbeckst]

yes – I noticed it when I was updating the docs for 2.0

[orbeckst]

Of course, we'll need to have a deprecated shim in it's old place until 3.0.

[orbeckst]

I'll just say that we want to do this and remove the proposal tag.

[orbeckst]

@ianmkenney please make sure that this issue is also on your TODO list. Thanks.