Thanks for the bug report and patch, but I ran into a problem with the 
patch. Suppose I do this:

emacs -Q
M-x electric-quote-mode
M-x customize-variable RET electric-quote-string, set it to t, save
M-x python-mode
'abc'

That is, I type apostrophe-a-b-c-apostrophe in Python mode. With the 
patch, the buffer will contain:

'abc’

The opening apostrophe is OK, but the closing apostrophe is considered 
to be inside a string so it is transmogrified into a right single 
quotation mark. A Python programmer would want both to be apostrophes.

I installed the attached patches into emacs-25 instead, as they should 
address this problem too. Please give it a try.