BF: pyproject.toml: Add setuptools to build-system requires

According to the failure on Travis:

    WARNING: The project does not specify a build backend, and pip
    cannot fall back to setuptools without 'setuptools>=40.8.0'

https://travis-ci.org/github/datalad/datalad/jobs/715984720#L1021

pyproject.toml

@@ -1,3 +1,3 @@
 [build-system]
 # wheel to get more lightweight (not EASY-INSTALL) entry-points
-requires = ["wheel"]
+requires = ["setuptools>=40.8.0", "wheel"]