gh-101178: Add Ascii85, Base85, and Z85 support to binascii (GH-102753)

Add Ascii85, Base85, and Z85 encoders and decoders to binascii,
replacing the existing pure Python implementations in base64.

This makes the codecs two orders of magnitude faster and consume
two orders of magnitude less memory.

Note that attempting to decode Ascii85 or Base85 data of length 1 mod 5
(after accounting for Ascii85 quirks) now produces an error, as no
encoder would emit such data. This should be the only significant
externally visible difference compared to the old implementation.

Co-authored-by: Serhiy Storchaka <[email protected]>