rex.notebook: pin numpy to 1.21.3

Newly released 1.21.4 is while trying to build a wheel. We can
reconsider when/if they add wheels for this version back.

Author: andreypopp

src/rex.notebook/setup.py

@@ -29,6 +29,7 @@
         "ipykernel == 5.1.2",
         "jupyter_client == 5.3.1",
         "papermill == 1.2.0",
+        "numpy == 1.21.3",
     ],
     entry_points={"rex.ctl": ["rex = rex.notebook.ctl"]},
     dependency_links=["https://dist.rexdb.org/packages/"],