Update kwarg for attr.ib to use 'converter' as 'convert' is due to be deprecated

src/_pytest/tmpdir.py

@@ -31,7 +31,7 @@ class TempPathFactory(object):
         # using os.path.abspath() to get absolute path instead of resolve() as it
         # does not work the same in all platforms (see #4427)
         # Path.absolute() exists, but it is not public (see https://bugs.python.org/issue25012)
-        convert=attr.converters.optional(
+        converter=attr.converters.optional(
             lambda p: Path(os.path.abspath(six.text_type(p)))
         )
     )