bpo-37388: Development mode check encoding and errors (GH-14341)

bpo-37388: Development mode check encoding and errors (GH-14341)

In development mode and in debug build, encoding and errors arguments
are now checked on string encoding and decoding operations. Examples:
open(), str.encode() and bytes.decode().

By default, for best performances, the errors argument is only
checked at the first encoding/decoding error, and the encoding
argument is sometimes ignored for empty strings.