BUG: JsonPointer.set() does not validate assignment target (RFC 6901 non-compliant)

[a-akinina]

Summary

JsonPointer.set() does not validate the assignment target, so it raises
TypeError or IndexError instead of JsonPointerException. Mirror of #70
on the write path; PR #76 fixes resolve, set stays unguarded.

Reproduction

from jsonpointer import JsonPointer

JsonPointer("/0").set("abc", {"v": 1}, inplace=False)
# TypeError: 'str' object does not support item assignment

JsonPointer("/0").set([], 42, inplace=False)
# IndexError: list assignment index out of range

JsonPointer("/5").set([1, 2], 42, inplace=False)
# IndexError: list assignment index out of range

Root cause

if isinstance(parent, Sequence) and part == '-':
    parent.append(value)
else:
    parent[part] = value  # str -> TypeError, list out of range -> IndexError

str is a Sequence, and there is no guard around the assignment.

Impact

jsonpatch catches IndexError but not TypeError, so JsonPatch.apply()
can raise TypeError on untrusted patch documents.

Environment

jsonpointer 3.1.1, Python 3.x, found by fuzzing