fix(nltk): remedy Windows temp-file problem (#3395)
**Summary**
The implementation of `tempfile.NamedTemporaryFile` on Windows Python is
problematic in certain situations.
In particular, it raises `PermissionError` when attempting to access the
temporary file by name rather than just by the file-descriptor returned
by the context-manager.
Remedy this situation by using `tempfile.TemporaryDirectory` instead and
using a file name of our choosing. The temporary directory is deleted
with all its contents when the context manager closes so the effect is
the same and does not produce the error on Windows.
