downgrade python 3.9 to 3.8 u code example

Example: how to downgrade python 3.9 to 3.8

$ brew unlink python
$ brew install --ignore-dependencies https://raw.githubusercontent.com/Homebrew/homebrew-core/e128fa1bce3377de32cbf11bd8e46f7334dfd7a6/Formula/python.rb
$ brew switch python 3.6.5
$ pip install tensorflow