Specify output_file encoding as utf-8 (#364)

Co-authored-by: Eric Brown <ericwb@users.noreply.github.com>
Co-authored-by: Luke Hinds <7058938+lukehinds@users.noreply.github.com>

bandit/cli/main.py

@@ -201,7 +201,7 @@ def main():
     )
     parser.add_argument(
         '-o', '--output', dest='output_file', action='store', nargs='?',
-        type=argparse.FileType('w'), default=sys.stdout,
+        type=argparse.FileType('w', encoding='utf-8'), default=sys.stdout,
         help='write report to filename'
     )
     group = parser.add_mutually_exclusive_group(required=False)