Set ReadOnlyDirectory attributes when fSeekKeys == 0 (scikit-hep#660)

* Set ReadOnlyDirectory attributes when fSeekKeys == 0

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

Co-authored-by: Martin <ka_hei_martin_kwok@brown.edu>
Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

src/uproot/reading.py

@@ -1405,7 +1405,8 @@ def __init__(self, path, cursor, context, file, parent):
         if self._fSeekKeys == 0:
             self._header_key = None
             self._keys = []
-
+            self._keys_lookup = {}
+            self._len = None
         else:
             keys_start = self._fSeekKeys
             keys_stop = min(keys_start + self._fNbytesKeys + 8, file.fEND)