Pin emmet core to fix task doc validation

Author: Jason Munro

setup.py

@@ -29,11 +29,11 @@
         "typing-extensions>=3.7.4.1",
         "requests>=2.23.0",
         "monty>=2021.3.12",
-        "emmet-core>=0.39.8",
+        "emmet-core<=0.50.0",
     ],
     extras_require={
         "all": [
-            "emmet-core[all]>=0.39.8",
+            "emmet-core[all]<=0.50.0",
             "custodian",
             "mpcontribs-client",
             "boto3"