fix PEP 518 configuration

Add `build-system.requires` key to `pyproject.toml`:
- this is necessary with pip 10.0.1, as otherwise the defaults will be
  to require both `setuptools` and `wheel` (and we only need the later)
- a `pyproject.toml` with no `build-system.requires` key will be invalid
  and rejected by a future version of pip
  (see pypa/pip#5416 (comment))

changelog.d/1404.misc.rst

@@ -0,0 +1 @@
+Fix PEP 518 configuration: set build requirements in ``pyproject.toml`` to ``["wheel"]``.

pyproject.toml

@@ -1,3 +1,6 @@
+[build-system]
+requires = ["wheel"]
+
 [tool.towncrier]
     package = "setuptools"
     package_dir = "setuptools"