Hi,

This patch tries to make `python-shell-send-block' more convenient.

I had added the function `python-shell-send-block'  twenty days ago in
bug#70609, it will send code block with header by default, and a
prefix "C-u" to call the function will send block without header.

And I received some feedback that sending a block without a header
should be a more used one.

Many python blocks begin with "if condition", and users may want to
send the block without its condition header to try different condition
branches.

So this patch will make sending block without header to be the default
behavior, and a prefix "C-u" will make the function to send block with
header.

Please help review this minor change. Thanks