[pyobj-] catch AttributeError viewing columns list

[midichef]

Viewing a sheet's columns with ^X sheet.columns gives an error trying to read formatted_help that causes all the attributes to fail to be displayed.

Traceback (most recent call last):
  File "/home/midichef/.venv/lib/python3.12/site-packages/visidata/pyobj.py", line 56, in PyobjColumns
    return [ColumnAttr(k, type=deduceType(getattr(obj, k))) for k in getPublicAttrs(obj)]
  File "/home/midichef/.venv/lib/python3.12/site-packages/visidata/pyobj.py", line 52, in getPublicAttrs
    return [k for k in dir(obj) if not k.startswith('_') and not callable(getattr(obj, k))]
  File "/home/midichef/.venv/lib/python3.12/site-packages/visidata/column.py", line 189, in formatted_help
    return MissingAttrFormatter().format(self.help, sheet=self.sheet, col=self, vd=vd)
AttributeError: 'ItemColumn' object has no attribute 'help'

A simpler way to solve this problem would be to replace self.help with getattr(self, 'help', ''), but that would break the guide formatting behavior. formatted_help was designed to be accessed when handling a format string for a guide: {sheet.cursorCol.formatted_help}". In that context, MissingAttrFormatter() looks for AttributeError to know when to fall back to displaying the format string unaltered:

visidata/visidata/utils.py

Lines 195 to 199 in 37f0a0c

	def get_field(self, field_name, args, kwargs):
	try:
	return super().get_field(field_name, args, kwargs)
	except (KeyError, AttributeError, IndexError, ValueError) as e:
	return ('{' + field_name + '}', field_name)

So this PR changes the way PyobjColumns accesses attributes, to be compatible with the MissingAttrFormatter design.