[3.14] gh-145264: Do not ignore excess Base64 data after the first padded quad (GH-145267) (GH-146326)

Base64 decoder (see binascii.a2b_base64(), base64.b64decode(), etc)
no longer ignores excess data after the first padded quad in non-strict
(default) mode.  Instead, in conformance with RFC 4648, it ignores the
pad character, "=", if it is present before the end of the encoded data.
(cherry picked from commit 4561f6418a691b3e89aef0901f53fe0dfb7f7c0e)