[3.12] gh-108487: Change assert that should've been DEOPT_IF (#108509)

* Remove assert that should've been DEOPT_IF

The assert(method != NULL) in CALL_NO_KW_LIST_APPEND is wrong --
this condition should lead to a deoptimization, and indeed there
is a DEOPT_IF two lines later that will trigger if method == NULL.

This would crash in a devious repro scenario (first seen live
in boto3 tests) when compiled with assertions enabled.
In a production version there is no crash, so impact is limited.

(The crash also appears in main; I will prepare a separate PR.)

* Add back a different assert(self != NULL)

* 📜🤖 Added by blurb_it.

---------

Co-authored-by: blurb-it[bot] <43283697+blurb-it[bot]@users.noreply.github.com>