I updated the python version from 3.6.4 to 3.6.5 today. This is because, in the process of distributing to Heroku, it recommends version 3.6.5. Therefore, the following power sh
You're trying to install ÿþpython-3.6.5 not python-3.6.5 as the console output suggests. Remove ÿþ and it should work as expected.
Heroku believes that your runtime.txt contains some extra characters:
ÿþpython-3.6.5
This is probably byte-order mark for a file encoded as UTF-16 in little-endian order. Make sure you're using a sane encoding for that file (and others). UTF-8 is a good choice in virtually all situations.