Improve `_nice_name` in `to.py`

[james-d-mitchell]

This function works pretty well but it swallows some information that's actually useful/required. For example,

In [3]: from libsemigroups_pybind11.to import _nice_name
In [4]: _nice_name(list[int])
Out[4]: 'list'

It'd be preferable if it returned list[int] here instead, since o/w one of the to functions error messages looks like this:

In [6]: from libsemigroups_pybind11 import *

In [7]: b1 = Bipartition([[1, -1], [2, -2], [3, -3], [4, -4]])
   ...: b2 = Bipartition([[1, -2], [2, -3], [3, -4], [4, -1]])
   ...: b3 = Bipartition([[1, -2], [2, -1], [3, -3], [4, -4]])
   ...: b4 = Bipartition([[1, 2], [3, -3], [4, -4], [-1, -2]])
   ...: S = FroidurePin(b1, b2, b3, b4)
In [10]: to(S, rtype=(Presentation,list[str]))
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
Cell In[10], line 1
----> 1 to(S, rtype=(Presentation,list[str]))

File ~/.local/share/mamba/envs/libsemigroups_pybind11_dev/lib/python3.13/site-packages/libsemigroups_pybind11/to.py:157, in to(rtype, *args)
    155 cxx_args = [_to_cxx(arg) for arg in args]
    156 if rtype not in _RETURN_TYPE_TO_CONVERTER_FUNCTION:
--> 157     raise TypeError(
    158         "expected the first keyword argument to be one of:"
    159         f"{_VALID_TYPES_STRING}"
    160         f"but found: {_nice_name(rtype)}"
    161     )
    162 constructor = rtype[0]
    163 return constructor(_RETURN_TYPE_TO_CONVERTER_FUNCTION[rtype](*cxx_args))

TypeError: expected the first keyword argument to be one of:
    * (Congruence, str)
    * (Congruence, list)
    * (FroidurePin)
    * (InversePresentation)
    * (InversePresentation, list)
    * (InversePresentation, str)
    * (KnuthBendix)
    * (KnuthBendix, Set, Order.shortlex)
    * (KnuthBendix, Trie, Order.shortlex)
    * (KnuthBendix, list, Set, Order.shortlex)
    * (KnuthBendix, list, Trie, Order.shortlex)
    * (KnuthBendix, str, Set, Order.shortlex)
    * (KnuthBendix, str, Trie, Order.shortlex)
    * (KnuthBendix, Set, Order.recursive)
    * (KnuthBendix, Trie, Order.recursive)
    * (KnuthBendix, list, Set, Order.recursive)
    * (KnuthBendix, list, Trie, Order.recursive)
    * (KnuthBendix, str, Set, Order.recursive)
    * (KnuthBendix, str, Trie, Order.recursive)
    * (Presentation)
    * (Presentation, str)
    * (Presentation, list)
    * (ToddCoxeter)
    * (ToddCoxeter, str)
    * (ToddCoxeter, list)
but found: (Presentation, list)