Relaxing `t-string` conversion restriction

[dg-pb]

@ericvsmith

Because format_spec is tied to a type, and conversion can be applied to any and every type.

Yes, this is how I see it as well.
Each serves its purpose.

And while t-string allows to pass-through any format_spec, conversion is restricted.

Of course, making things restricted is safer in general, but the cost seems reasonable:

1. If implemented it just falls into the same category as format_spec. i.e. say user has his own conversion t implemented, stdlib implements t, user needs to change the letter. But it is already the same with format_spec, so I think it adds more value than causes actual problems. e.g. how many new conversions were added to stdlib in last 10 years?
2. string.Formatter has been allowing any character for a long time now and I don’t think there were any issues.

Thus, I think this is fairly safe.