Move build requirements from setup.cfg to pyproject.toml.

Using `setup_requires` in `setup.cfg` is deprecated in newer verisons of
`setuptools`, in favor of PEP517-style `[build-system].requires` in
`pyproject.toml`.

More concretely, I believe moving to the newer style will fix an issue
my team is hitting in our monorepo, where we've occasionally been seeing
failures to install `cairocffi` because of "file already exists" errors
when building the underlying `cffi`. I found a discussion in another
project where the `cffi` maintainer said the issue arises when the same
dependency is listed in both `setup_requires` and `install_requires`
(pypa/cibuildwheel#813 (comment))
and then confirmed that switching to a PEP517-style build fixed things
(pypa/cibuildwheel#813 (comment)).

pyproject.toml

@@ -0,0 +1,3 @@
+[build-system]
+requires = ["setuptools", "cffi >= 1.1.0"]
+build-backend = "setuptools.build_meta"

setup.cfg

@@ -34,9 +34,6 @@ project_urls =
 
 [options]
 packages = find:
-setup_requires =
-  cffi >= 1.1.0
-  setuptools
 install_requires =
   cffi >= 1.1.0
 python_requires = >= 3.7