fix numpy version at compile time

tools/pylib/_boutcore_build/setup.cfg.in

@@ -42,8 +42,9 @@ setup_requires =
     cython
     numpy
 install_requires =
-    cython
-    numpy
+EOF
+echo "    numpy==$(python -c 'from numpy.version import version;print(version)')"
+cat << "EOF"
 tests_require =
     pytest >= 3.3.0
 include_package_data = True