test: add 3.10 unit tests (#69)

* test: add 3.9, 3.10 unit tests

* chore: add 3.10 constraints

Author: busunkim96

packages/google-cloud-testutils/noxfile.py

@@ -74,7 +74,7 @@ def lint_setup_py(session):
     session.run("python", "setup.py", "check", "--restructuredtext", "--strict")
 
 
-@nox.session(python=["3.6", "3.7", "3.8", "3.9"])
+@nox.session(python=["3.6", "3.7", "3.8", "3.9", "3.10"])
 def unit(session):
     constraints_path = str(
         CURRENT_DIRECTORY / "testing" / f"constraints-{session.python}.txt"
@@ -119,4 +119,4 @@ def update_lower_bounds(session):
         "google-cloud-testutils",
         "--constraints-file",
         "testing/constraints-3.6.txt",
-    )
+    )