feat: remove multipart support

It is broken since Python 3 and is unused.

It is difficult to maintain support for this with CSRF checks.
4 jobs for !23 with topic/default/remove-multipart-support in 4 minutes and 4 seconds (queued for 46 seconds)
latest detached
Status Name Job ID Coverage
  Lint
passed black #357432

00:00:28

passed check-manifest #357433

00:00:55

passed flake8 #357431

00:01:21

 
  Tests
passed py3 #357434

00:01:47