deprecate scrapy.utils.multipart

It was a shim for w3lib.form, but w3lib.form is deprecated as well.

scrapy/utils/multipart.py

@@ -3,5 +3,13 @@ Transitional module for moving to the w3lib library.
 
 For new code, always import from w3lib.form instead of this module
 """
+import warnings
 
+from scrapy.exceptions import ScrapyDeprecationWarning
 from w3lib.form import *
+
+
+warnings.warn("Module `scrapy.utils.multipart` is deprecated. "
+              "If you're using `encode_multipart` function, please use "
+              "`urllib3.filepost.encode_multipart_formdata` instead",
+              ScrapyDeprecationWarning, stacklevel=2)