Push failed when deploying on heroku [duplicate]

Question

-----> Building on the Heroku-20 stack
-----> Determining which buildpack to use for this app
-----> Python app detected
-----> Using Python version specified in runtime.txt
Traceback (most recent call last):
  File "/tmp/codon/tmp/buildpacks/0f40890b54a617ec2334fac0439a123c6a0c1136/vendor/runtime-fixer", line 8, in <module>
    r = f.read().strip()
  File "/usr/lib/python3.8/codecs.py", line 322, in decode
    (result, consumed) = self._buffer_decode(data, self.errors, final)
UnicodeDecodeError: 'utf-8' codec can't decode byte 0xff in position 0: invalid start byte
/tmp/codon/tmp/buildpacks/0f40890b54a617ec2334fac0439a123c6a0c1136/bin/steps/python: line 5: warning: command substitution: ignored null byte in input
 !     Requested runtime (��python-3.8.10
 !     Aborting.  More info: https://devcenter.heroku.com/articles/python-support
 !     Push rejected, failed to compile Python app.
 !     Push failed

my requirements.txt

aiohttp==3.7.4.post0
async-timeout==3.0.1
attrs==21.4.0
chardet==4.0.0
discord==1.7.3
discord.py==1.7.3
idna==3.3
multidict==6.0.2
typing_extensions==4.1.1
yarl==1.7.2

runtime.txt

python-3.8.10

I have tried different ways but never worked. It says failed to compile. Please help me to do this.