Graphviz is only needed for docs build

.github/workflows/test.yml

@@ -1,35 +1,31 b''
 name: Run tests
 
 on: [push]
 
 jobs:
   test:
     runs-on: ubuntu-latest
     strategy:
       matrix:
         python-version: [3.7, 3.8, 3.9]
 
     steps:
     - uses: actions/checkout@v2
     - name: Set up Python ${{ matrix.python-version }}
       uses: actions/setup-python@v2
       with:
         python-version: ${{ matrix.python-version }}
-    - name: Install Graphviz
-      run: |
-        sudo apt-get update 
-        sudo apt-get install graphviz
     - name: Install and update Python dependencies
       run: |
         python -m pip install --upgrade pip
         python -m pip install --upgrade setuptools
         python -m pip install --upgrade -e file://$PWD#egg=ipython[test]
         python -m pip install --upgrade --upgrade-strategy eager trio curio 
         python -m pip install --upgrade pytest pytest-trio 'matplotlib!=3.2.0'
         python -m pip install --upgrade codecov check-manifest pytest-cov anyio
     - name: iptest
       run: |
         cd /tmp && iptest --coverage xml && cd -
     - name: pytest
       run: |
         pytest