fix: rollback six to 1.15 (#391)

* fix: remove grpcio and rollback six to 1.15

* fix: add grpcio 1.34 back

setup.py

@@ -32,7 +32,7 @@
 tensorboard_extra_require = [
     "tensorflow-cpu>=2.3.0, <=2.5.0",
     "grpcio~=1.34.0",
-    "six~=1.16.0",
+    "six~=1.15.0",
 ]
 metadata_extra_require = ["pandas >= 1.0.0"]
 full_extra_require = tensorboard_extra_require + metadata_extra_require