chore(python): make string_repr private

[MarcoGorelli]

As reported on Discord:

>>> pl.Int16.string_repr()
TypeError: DataType.string_repr() missing 1 required positional argument: 'self'

[stinodego]

To be honest, that class should be private. It should only be used internally for printing dataframes. Use str(pl.Int16) if you want a string representation.

[MarcoGorelli]

ok yeah, if it's only meant for internal usage, that's probably better

is it worth going through a deprecation cycle, given it currently errors anyway for non-nested types (and so wouldn't reach the warning anyway)?

[stinodego]

I don't think we have to deprecate it, we can just make it private, seeing it was broken anyway.

Pinging @ritchie46 to confirm - maybe he sees this differently.

[ritchie46]

Yeah, we can just make this private. If its not in our ref-guide it is not public. :)