build: use static dependency metadata

Closes #256.

Author: dimbleby

pyproject.toml

@@ -21,14 +21,21 @@ classifiers = [
     "Programming Language :: Python :: 3.14",
 ]
 keywords = ["zstandard", "zstd", "compression"]
+dependencies = []
+
+[project.optional-dependencies]
+cffi = [
+    'cffi>=1.11 ; python_version < "3.13" and platform_python_implementation != "PyPy"',
+    'cffi>=1.17 ; python_version >= "3.13" and platform_python_implementation != "PyPy"',
+]
 
 [project.urls]
 Homepage = "https://github.com/indygreg/python-zstandard"
 Documentation = "https://python-zstandard.readthedocs.io/en/latest/"
 
 [build-system]
 requires = [
-    "cffi>=1.17.0",
+    "cffi>=1.17.0 ; platform_python_implementation != 'PyPy'",
     "packaging",
     "setuptools",
 ]

setup.py

@@ -158,13 +158,5 @@
     ext_modules=extensions,
     cmdclass={"build_ext": setup_zstd.RustBuildExt},
     test_suite="tests",
-    install_requires=[
-        # cffi is required on PyPy.
-        "cffi>=%s; platform_python_implementation == 'PyPy'"
-        % MINIMUM_CFFI_VERSION
-    ],
-    extras_require={
-        "cffi": ["cffi>=%s" % MINIMUM_CFFI_VERSION],
-    },
     tests_require=["hypothesis"],
 )