fix: add missing imports so that dataframes can be written to ttrees (#1524)

Added missing imports

Author: ariostas

src/uproot/writing/writable.py

@@ -37,6 +37,8 @@
 import uproot.models.TObjString
 import uproot.sink.file
 import uproot.writing._cascade
+import uproot.writing._cascadetree
+import uproot.writing.identify
 from uproot._util import no_filter, no_rename