Due to the complexity of the dependencies, PyAV is not always the easiest Python package to install. The most straight-foward install is via conda-forge:
conda install av -c conda-forge
See the Conda quick install docs to get started with (mini)Conda.
If you want to use your existing FFmpeg/Libav, the C-source version of PyAV is on PyPI:
pip install av
And if you want to build from the absolute source (for development or testing):
git clone firstname.lastname@example.org:mikeboers/PyAV cd PyAV source scripts/activate make
Have fun, Read the Docs, and good luck!