Use warnings.warn instead of unused variable _logger

After adding a type signature to create_file_node(), mypy complained
that _logger was undefined.  That was correct, as I'd copied and pasted
that section from DFXML's walk_to_dfxml.py.

Another issue around a multi-value-types dictionary will require a
bigger patch before the type signature gets committed.

References:
* [AC-211] Add static type checking to CASE-Utilities-Python

Signed-off-by: Alex Nelson <alexander.nelson@nist.gov>

Author: ajnelson-nist

case_utils/case_file/__init__.py

@@ -21,6 +21,7 @@
 import hashlib
 import os
 import typing
+import warnings
 
 import rdflib  # type: ignore
 
@@ -166,7 +167,7 @@ def create_file_node(graph, filepath, node_iri=None, node_prefix=DEFAULT_PREFIX,
             raise ValueError("Failed to confirm hashes of file %r." % filepath)
         if successful_hashdict["filesize"] != file_stat.st_size:
             # TODO - Discuss with AC whether this should be something stronger, like an assertion error.
-            _logger.warning(
+            warnings.warn(
               "Inode file size and hashed file sizes disagree: %d vs. %d.",
               file_stat.st_size,
               successful_hashdict["filesize"]