Add Python 3.15 base64 and binascii updates

[JelleZijlstra]

Summary:

• Add Python 3.15 base64 and binascii codec parameter updates.
• Add new public binascii alphabet constants.
• Remove fixed 3.15 stubtest allowlist entries, keeping the b64decode sentinel-default false positive documented.

Relevant report entries:

• Added base64/binascii alphabet constants, base32/base64/base85 codec helpers, expanded codec parameters, and ignorechars for hex decoding (CPython PRs: Option to skip padding for base64 urlsafe encoding/decoding cpython#73613, Support multiline output in binascii.b2a_base64() and base64.b64encode() cpython#143214, Add the ignorechars parameter in the Base64 decoder cpython#144001, binascii base64 & base32 decode ignore excess padding bits in their input cpython#146311, base64.z85encode should have "pad" parameter cpython#143103, C accelerator for Base32 character encoding cpython#146192, base64.b85encode uses significant amount of RAM cpython#101178, Support alternative alphabets in BaseXX encodings cpython#145980; source: https://github.com/python/cpython/blob/main/Modules/binascii.c)

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

pandas (https://github.com/pandas-dev/pandas)
+ pandas/core/computation/ops.py:328: error: Need type annotation for "_binary_ops_dict" (hint: "_binary_ops_dict: dict[<type>, <type>] = ...")  [var-annotated]