This is useful for alpine linux containers, to avoid needing a
multistage build to build + install the lxml package.

I tested it by building using make, then installing and using the
package in an alpine linux container:

```bash
❯ make wheel_musllinux_1_1_x86_64

❯ docker run \
  --rm \
  --workdir /tmp/workdir \
  --volume="$PWD:/tmp/workdir" \
  -t alpine \
  sh -c "
  set -e
  apk add python3
  # virtualenv
  python3 -m venv ~/.venv
  . ~/.venv/bin/activate
  # need a more recent version of pip for manylinux wheels
  pip install pip==21.2.4
  pip install wheelhouse/musllinux_1_1_x86_64/lxml-4.6.3-cp39-cp39-musllinux_1_1_x86_64.whl
  python -c 'import lxml; print(lxml.__version__)'
  "