bpo-43086: Add handling for out-of-spec data in a2b_base64 (GH-24402)

binascii.a2b_base64 gains a strict_mode= parameter. When enabled it will raise an
error on input that deviates from the base64 spec in any way.  The default remains
False for backward compatibility.

Code reviews and minor tweaks by: Gregory P. Smith <greg@krypto.org> [Google]