Not supported in Pandas >= 3.0.0

An exception is thrown when writing a ply file. The issue comes from trying to index the pandas dataframe dtypes by index: `df.dtypes[i]`. It only supports indexing by name now.

Exception:
```
.venv/lib/python3.12/site-packages/pyntcloud/core_class.py:182: in to_file
TO_FILE[ext](**kwargs)
.venv/lib/python3.12/site-packages/pyntcloud/io/ply.py:209: in write_ply
header.extend(describe_element('vertex', points))
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
.venv/lib/python3.12/site-packages/pyntcloud/io/ply.py:260: in describe_element
f = property_formats[str(df.dtypes[i])[0]]
^^^^^^^^^^^^
.venv/lib/python3.12/site-packages/pandas/core/series.py:959: in __getitem__
return self._get_value(key)
^^^^^^^^^^^^^^^^^^^^
.venv/lib/python3.12/site-packages/pandas/core/series.py:1046: in _get_value
loc = self.index.get_loc(label)
^^^^^^^^^^^^^^^^^^^^^^^^^
```

Versions:
- pyntcloud: 0.3.1
- pandas: 3.0.0