Add optional dependency on pyvips-binary

Allowing users to install pyvips with binary packages via:
pip install "pyvips[binary]"

See: https://pypi.org/project/pyvips-binary/.

.github/workflows/ci.yml

@@ -1,6 +1,6 @@
 name: CI
 
-on: [ push, pull_request ]
+on: [push, pull_request]
 
 jobs:
   CI:

pyproject.toml

@@ -73,6 +73,7 @@ exclude = [
 ]
 
 [project.optional-dependencies]
+binary = ["pyvips-binary"]
 # All the following are used for our own testing
 tox = ["tox"]
 test = [