Remove homebrew dependency in macOS CI workflow

The GitHub documentation states that python3 w/ pip3 is already
installed, and homebrew is slow as molasses (and when it finally runs it
gives a warning about python already being installed and up to date).

.github/workflows/main.yml

@@ -116,7 +116,6 @@ jobs:
     - uses: actions/checkout@v2
     - name: Install deps
       run: |
-        brew install python3
         sudo pip3 install pexpect
     - name: cmake
       run: |