Add support for musl wheels

CONTRIBUTING.rst

@@ -54,7 +54,7 @@ Here's a brief check list for releasing a new version:
   CI build takes a while.  These wheels are not automatically uploaded,
   but there's ``./bin/download-windows-wheels`` script that downloads built
   wheels.  Then upload them with ``twine``.
-- Run ``./bin/build-manylinux-wheels`` to build linux wheels and upload them to
+- Run ``./bin/build-linux-wheels`` to build linux wheels and upload them to
   PyPI (takes ~5 minutes).
 - The `docs website`__ also has to be updated.
   It's currently a static website deployed on GitHub Pages.

bin/build-manylinux-wheels → bin/build-linux-wheels

@@ -21,17 +21,17 @@ def check_call(*cmd):
 
 def main():
     os.makedirs('dist', exist_ok=True)
-    for python in ('cp27-cp27mu', 'cp36-cp36m'):
+    for platform in ('manylinux1', 'musllinux_1_1'):
         with tempfile.TemporaryDirectory() as work:
-            pip = f'/opt/python/{python}/bin/pip'
+            pip = '/opt/python/cp36-cp36m/bin/pip'
             check_call(
                 'docker', 'run', '-ti',
                 # Use this so the files are not owned by root
                 '--user', f'{os.getuid()}:{os.getgid()}',
                 # We'll do building in /work and copy results to /dist
                 '-v', f'{work}:/work:rw',
                 '-v', '{}:/dist:rw'.format(os.path.abspath('dist')),
-                'quay.io/pypa/manylinux1_x86_64:latest',
+                f'quay.io/pypa/{platform}_x86_64:latest',
                 'bash', '-exc',
                 '{} wheel --verbose --wheel-dir /work --no-deps libsass && '
                 'auditwheel repair --wheel-dir /dist /work/*.whl'.format(pip),