Assuming some python buffer like:

#....
if 1:
    print 1

where the selected region is "print \"1\"" (mark before "p"),
`python-shell-send-region' would fail with:

File "test.py", line 3
    print 1
        ^
IndentationError: expected an indented block

The reason lies within `python-shell-buffer-substring', which assumes
that a region always starts with the correct indentation.

The attached patch fixes the above behavior: when the region doesn't
start with indentation, but the current block does (looking at
(current-indentation)), we prefix the fillstr with the correct amount of
indent.