Add comment to explain why we skip PyPy Windows amd64

pyproject.toml

@@ -22,7 +22,10 @@ skip = [
     # are using conda to install our external shared library dependencies.
     "*i686*", "*musl*",
     # Skip these platforms because I haven't worked out the compiler config.
-    "*win32", "pp*-win_amd64",
+    "*win32",
+    # Skip this platform because meson cannot find numpy-config.
+    # See https://github.com/nasa-gcn/hpx/pull/12
+    "pp*-win_amd64",
 ]
 before-all = [
     "curl -L https://micro.mamba.pm/install.sh | bash",