[parquet] stringify source to handle both URLs and local paths

Stringify the source path that we feed into `pq.read_table`. This
produces consistent results for local paths and URLs, where using a
`visidata.Path` directly tries to load the underlying `pathlib.Path`
object at `_path`.

visidata/loaders/parquet.py

@@ -16,7 +16,7 @@ def iterload(self):
  pq = vd.importExternal('pyarrow.parquet', 'pyarrow')
  from visidata.loaders.arrow import arrow_to_vdtype
 
- self.tbl = pq.read_table(self.source)
+ self.tbl = pq.read_table(str(self.source))
  self.columns = []
  for colname, col in zip(self.tbl.column_names, self.tbl.columns):
  c = ParquetColumn(colname,