Replaced deprecated *textbox* method with updated *textbbox* method

[realasib]

The textsize method has been deprecated in newer version of Pillow

I tried to use the script to generate some certificate but it was not working as instructed. Then I found out that the textsize method has been deprecated and replaced by the textbbox method in newer versions of Pillow. So, I changed the code according to it. Hope it would get accepted!