python-shell-send-string changes break continued Python commands

[JD Smith]

I’ve noticed some breaking changes to the function python-shell-send-string in 
the recent past, related to continued commands.  

Example of a continued Python command:

>>> for i in range(12):
… 

I maintain a package called python-mls [1] which enables sending multi-line 
statements to the inferior python process.  It works by noticing the “…” 
continuation prompt python provides on continued commands, interrupting, and 
allowing Emacs to format and send its own multiline command (check the link for 
a GIF of it in action).

Unfortunately, the new logic of `python-shell-send-string’ defeats all 
continued commands, since it wraps even simple single line statements like “if 
True:” in the helper function __PYTHON_EL_eval().  This helper will attempt to 
compile the statement, breaking “continued commands” as above.  

Single-line commands do not need any special treatment when sending to the 
python interpreter, and are likely the most common type of command sent.

To reenable continued command functionality as well as for efficiency, simply 
using `comint-send-string' for single-line command input, as before, would be a 
suitable fix.   Pseudo-code:

(if (string-match ".\n+." string)   ;Multiline.
    (new-PYTHON_EL_eval_functionality…) 
  (comint-send-string process string))

I’m happy to put a patch together if this sounds sensible. 

[1] https://github.com/jdtsmith/python-mls