Pin the lower bound of PEP517-required setuptools (#171)

* Pin the lower bound of PEP517-required setuptools

It fails with Pip's buggy build env isolation combined with the
default fallback for the build backend which is
``setuptools.build_meta.__legacy__``.

* Point to setuptools PEP 517 backend explicitly

pyproject.toml

@@ -1,7 +1,10 @@
 [build-system]
 # Must be kept in sync with `setup_requirements` in `setup.py`
 requires = [
-    "setuptools",
+    "setuptools>=40.8.0",
     "wheel",
     "cffi>=1.1; python_implementation != 'PyPy'",
 ]
+# Point to the setuptools' PEP517 build backend explicitly to
+# disable Pip's fallback guessing
+build-backend = "setuptools.build_meta"