How to downgrade Homebrew Python to 3.6
A new Python version has been released, but TensorFlow does not yet support it on account of a breaking change with async and await being made reserved keywords. Homebrew has updated their default python version to 3.7, and it is quite difficult to find out how to downgrade. To save headaches, the answer is that you need to install a the python formula from a version prior to the update:
$ brew unlink python
$ brew install https://raw.githubusercontent.com/Homebrew/homebrew-core/e128fa1bce3377de32cbf11bd8e46f7334dfd7a6/Formula/python.rb
$ brew switch python 3.6.5
You can now python -m pip install tensorflow to your hearts content.