How to parse raw HTTP POST form data in Python?

Question

I have a string like this:

form_data = '--BoUnDaRyStRiNg
Content-Disposition: form-data; name="arg"

1
--BoUnDaRyStRiNg
Content-Disposition: form-data; name="email"

somebody@example.com
--BoUnDaRyStRiNg
Content-Disposition: form-data; name="some_other_value"

333
--BoUnDaRyStRiNg--
'

And it's content type:

content_type = 'multipart/form-data; boundary=BoUnDaRyStRiNg'

How do I parse this string into a dictionary without writing a custom parser?

data_dict = {'arg': '1', 'email': 'somebody@example.com', 'some_other_value': '333'}