Issue #19619: Blacklist non-text codecs in method API

str.encode, bytes.decode and bytearray.decode now use an
internal API to throw LookupError for known non-text encodings,
rather than attempting the encoding or decoding operation and
then throwing a TypeError for an unexpected output type.

The latter mechanism remains in place for third party non-text
encodings.

Backported changeset d68df99d7a57.
#20740: desquarify 2.
#20628: make it clear that DictReader/Writer *fieldnames* is a Sequence.

Patch by Sean Rodman.
#20628: wrap lines to < 80.