Add Python 3.15 JSON and sqlite3 updates

[JelleZijlstra]

Summary:

• Add Python 3.15 JSON array_hook support.
• Update _json.scanstring's 3.15 signature.
• Add Python 3.15 sqlite3 keyword and callback signature updates.
• Remove fixed 3.15 stubtest allowlist entries.

Relevant report entries:

• Added json.load and json.loads array_hook (CPython PR: Add "array_hook" parameter to JSON decoder cpython#146440; source: https://github.com/python/cpython/blob/main/Lib/json/__init__.py)
• Added json.decoder.JSONDecoder(array_hook=...) and _json.make_scanner.array_hook; changed _json.scanstring to the 3.15 positional-only pystr, end, strict signature (CPython source: https://github.com/python/cpython/blob/e81025e6d2e01e38c5a5b656af5739ac9df5ff55/Lib/json/decoder.py, https://github.com/python/cpython/blob/e81025e6d2e01e38c5a5b656af5739ac9df5ff55/Modules/_json.c)
• Added sqlite3.SQLITE_KEYWORDS and _sqlite3.SQLITE_KEYWORDS; updated several sqlite3.Connection callback registration methods to positional-only parameters (CPython source: https://github.com/python/cpython/blob/e81025e6d2e01e38c5a5b656af5739ac9df5ff55/Modules/_sqlite/module.c, https://github.com/python/cpython/blob/e81025e6d2e01e38c5a5b656af5739ac9df5ff55/Modules/_sqlite/clinic/connection.c.h)

[github-actions]

According to mypy_primer, this change has no effect on the checked open source code. 🤖🎉