Relaxing `t-string` conversion restriction

[dg-pb]

@dg-pb

However, for implementing more advanced features, one will inevitably have to venture into format_spec. E.g.:

1. Multiple conversions
2. Parameterised conversions

This has left me thinking.
Maybe it isn’t worth doing any-single-character permission at all then.

Maybe then for this to be properly useful it needs to allow many characters. i.e.

1. After ! take any single character (backwards compatible)
2. Then take until not : or }

Could then do things like:

'{!tnr}', which would:
type(obj) -> .__name__ -> repr

Consistency aside, allowing any single character doesn’t seem to help that much anymore to me…