MAINT Fix minimal numpy dependency in pyproject.toml (#51)

Author: rth

pyproject.toml

@@ -1,6 +1,20 @@
 [build-system]
 # build with the oldest numpy that has pre-build wheels for Py3.7 on PyPi
-requires = ["setuptools", "wheel", "cython>=0.28", "numpy==1.14.5"]
+requires = [
+    "setuptools",
+    "wheel",
+    "cython>=0.28",
+    "numpy==1.14.5; python_version=='3.5' and platform_system!='AIX'",
+    "numpy==1.14.5; python_version=='3.6' and platform_system!='AIX'",
+    "numpy==1.14.5; python_version=='3.7' and platform_system!='AIX'",
+    "numpy==1.17.3; python_version>='3.8' and platform_system!='AIX'",
+    # Minimum supported numpy 1.16 for AIX
+    # see https://github.com/scipy/scipy/pull/10431
+    "numpy==1.16.0; python_version=='3.5' and platform_system!='AIX'",
+    "numpy==1.16.0; python_version=='3.6' and platform_system=='AIX'",
+    "numpy==1.16.0; python_version=='3.7' and platform_system=='AIX'",
+    "numpy==1.17.3; python_version>='3.8' and platform_system=='AIX'",
+]
 
 [tool.black]
 line-length = 79