gh-146311: Reject non-canonical padding bits in base32, 64, & 85 decoding (GH-146312)

gh-146311: Reject non-canonical padding bits in base32, 64, & 85 decoding (GH-146312)

Add `canonical=False` keyword argument to `a2b_base64`, `a2b_base32`, `a2b_base85`, and `a2b_ascii85` (and their `base64` module wrappers). When `canonical=True`, non-canonical encodings are rejected per [RFC 4648 section 3.5](https://datatracker.ietf.org/doc/html/rfc4648.html#section-3.5).

This is independent of `strict_mode`.

For base85/ascii85, the check also rejects single-character final groups (never produced by a conforming encoder) and verifies partial group padding matches what the encoder would produce.

Co-authored-by: Serhiy Storchaka via lots of great code review!