The patch seems broken to me.

In cgi.parse_multipart(), the 'boundary' variable can be a string even though it is concatenated to bytes. Its default value is a string, and a string can be given via the pdict argument. There is no validity check other than valid_boundary(), which allows both string and bytes.

Most of the changes to test_cgi.py are entirely unrelated. The one test added which tests cgi.parse_multipart() should fail since it uses a string (not bytes) boundary, while the correct boundary for the test is commented out.

I short this patch seems half-baked. IMO reject this patch and fix just the bytes/strings issue with cgi.parse_multipart. Or, as mentioned in the comments, use FieldStorage to implement it and be done with it.