Python 3.6 causes error, python 3.5 does not.

[Jim]

Mint 18
Libreoffice 5.1.6.2
Python 3.6.5 in one virtual environment
Python 3.5.2 in another

I am writing a script that uses pyautogui to get some data and paste it 
into a Libreoffice calc file, there by bypassing the complexity of uno.

The problem is it runs fine if I use python 3.5. If I use python 3.6 it 
opens the calc file then pops up a dialog saying "std::bad_alloc". There 
are no relevant errors in the terminal. At this point I must reload the 
file and let calc recover it. I googled "std::bad_alloc" but didn'

This is the portion of the code that causes the error:

import pyautogui
import subprocess
import threading
import time

LONG_ENOUGH = 5

class LO():
     '''Manipulate libreoffice not using UNO'''
     def __init__(self):
         self.filename = 
'/home/jfb/Documents/Financial/Investing/Stocks.ods'
         opener = threading.Thread(target=self.open_it, 
args=(self.filename,))
         opener.start()
         time.sleep(5) #LONG_ENOUGH)
         #self.manipulate_LO()

     #Open and work with libreoffice
     def open_it(self, filename):
         #subprocess.call(["libreoffice", filename])
         subprocess.run(["libreoffice", filename])

lo = LO()

To complicate matters not all .ods files show this problem. I ran some 
tests. Two of my .ods files are effected but a couple of others are not. 
A new test file I created is not effected.

I don't know if I have a Libreoffice problem, file corruption or a 
Python problem. The fact that 3.6 gives an error but 3.5 does not is the 
reason I decided to ask here first.

Regards,  Jim