Pin Numpy version to <2

This will help with publishing a repodata patch for conda-forge as per
<#688 (comment)>.
This will no longer be necessary when we are finished with
<#689>.

pyproject.toml

@@ -48,7 +48,7 @@ keywords = [
 dependencies = [
     "setuptools>=48.0.0",
     "scipy>=0.14",
-    "numpy>=1.17.0",
+    "numpy>=1.17.0,<2",
     "filelock",
     "etuples",
     "logical-unification",