Notify users of new versions of cellxgene (#1078)

* Notify users of new versions of cellxgene

Fixes https://github.com/chanzuckerberg/cellxgene/issues/683

* Do not use PyGithub client

* Protect against AttributeError

* Document that all version tags must follow SemVer

* Release tags `should -> MUST` follow semantic versioning

server/test/test_api.py

@@ -19,7 +19,17 @@ class EndPoints(unittest.TestCase):
 
     @classmethod
     def setUpClass(cls):
-        cls.ps = Popen(["cellxgene", "launch", "../example-dataset/pbmc3k.h5ad", "--verbose", "--port", "5005"])
+        cls.ps = Popen(
+            [
+                "cellxgene",
+                "--no-upgrade-check",
+                "launch",
+                "../example-dataset/pbmc3k.h5ad",
+                "--verbose",
+                "--port",
+                "5005",
+            ]
+        )
         session = requests.Session()
         for i in range(90):
             try: