[Python-3000] PEP-3125 -- remove backslash continuation

[Andrew Koenig]

> I am worried that (as no indent is required on the next line) it will
> accidentally introduce legal interpretations for certain common (?)
> typos, e.g.

>   x = y+    # Used to be y+1, the 1 got dropped
>   f(x)

A reasonable worry.  It could still be solved at the lexical level by
requiring every continuation line to have more leading whitespace than the
first of the lines being continued, and still not mapping that whitespace
into an INDENT, but of course that approach adds complexity.

All I can say is that it worked in practice in EFL, and I adopted the same
approach in Snocone without any complaints.  (Of course Python has lots more
users than Snocone)