make it possible to compile from sources against numpy on macOS aarch64 (#475)

pyproject.toml

@@ -3,6 +3,7 @@ requires = [
   "setuptools", "wheel", "scikit-build", "cmake", "pip",
   "numpy==1.13.3; python_version=='3.6' and platform_machine != 'aarch64'",
   "numpy==1.19.3; python_version>='3.6' and sys_platform == 'linux' and platform_machine == 'aarch64'",
+  "numpy==1.20.1; python_version>='3.6' and sys_platform == 'darwin' and platform_machine == 'aarch64'",
   "numpy==1.14.5; python_version=='3.7' and platform_machine != 'aarch64'",
   "numpy==1.17.3; python_version=='3.8' and platform_machine != 'aarch64'",
   "numpy==1.19.3; python_version>='3.9' and platform_machine != 'aarch64'"

setup.py

@@ -31,10 +31,14 @@ def main():
     if sys.version_info[:2] >= (3, 9):
         minimum_supported_numpy = "1.19.3"
 
-    # arm64 is a special case
-    if sys.version_info[:2] >= (3, 6) and platform.machine() == "aarch64":
+    # linux arm64 is a special case
+    if sys.platform.startswith("linux") and sys.version_info[:2] >= (3, 6) and platform.machine() == "aarch64":
         minimum_supported_numpy = "1.19.3"
 
+    # macos arm64 is a special case
+    if sys.platform == "darwin" and sys.version_info[:2] >= (3, 6) and platform.machine() == "aarch64":
+        minimum_supported_numpy = "1.20.1"
+
     numpy_version = "numpy>=%s" % minimum_supported_numpy
 
     python_version = cmaker.CMaker.get_python_version()