Ensure setuptools.build_meta is available

pquentin · pquentin · commit 8dd4ae26fbb7 · 2018-11-26T11:52:25.000+04:00
It looks it is required by tox, but is only available in the latest
setuptools release.
diff --git a/pyproject.toml b/pyproject.toml
@@ -1,3 +1,3 @@
 [build-system]
-requires = ["setuptools", "wheel", "unasync"]
+requires = ["setuptools>=40.6.2", "wheel", "unasync"]
 build-backend = "setuptools.build_meta"
