If you change the corresponding code to:
elif inkey <= 127:
    if pygame.key.get_mods() & KMOD_SHIFT or  pygame.key.get_mods() & KMOD_CAPS: # if shift is pressed  or caps is on
        current_string.append(chr(inkey).upper()) # make string uppercase
    else:
        current_string.append(chr(inkey)) # else input is lower 
That should work.
If you want more info on keyboard modifier states look here
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solved How to allow caps in this input box program for pygame?