As noted in the comments of #22758 by Georg Brandle: 

* Django uses __init__(str()) roundtripping, which is not explicitly supported by the library, and worked by accident with previous versions.  That it works again with 3.3+ is another accident, and a bug.

(The change for #16611 reintroduces "lax" parsing behavior that the security fix [1] was supposed to prevent.)

[1] https://hg.python.org/cpython/rev/d3663a0f97ed