Use PEP 518 pyproject.toml file to manage build dependencies

MANIFEST.in

@@ -1,6 +1,7 @@
 include CMakeLists.txt
 include LICENSE
 include schemas.xml
+include pyproject.toml
 include openmc/data/reconstruct.pyx
 include docs/source/_templates/layout.html
 include docs/sphinxext/LICENSE

pyproject.toml

@@ -0,0 +1,2 @@
+[build-system]
+requires = ["setuptools", "wheel", "numpy", "cython"]

tools/ci/travis-install.sh

@@ -7,10 +7,6 @@ set -ex
 # Upgrade pip before doing anything else
 pip install --upgrade pip
 
-# Running OpenMC's setup.py requires numpy/cython already
-pip install numpy
-pip install cython
-
 # pytest installed by default -- make sure we get latest
 pip install --upgrade pytest