chore: tweak 'python_requires' to match supported Python versions (#114)

* chore: tweak 'python_requires' to match supported Python versions

Release-As: 1.24.0

* tests: drop Python 3.5 for unit tests
diff --git a/noxfile.py b/noxfile.py
index 3f01ef1..86758e1 100644
--- a/noxfile.py
+++ b/noxfile.py
@@ -71,13 +71,13 @@
         session.run(*pytest_args)
 
 
-@nox.session(python=["2.7", "3.5", "3.6", "3.7", "3.8", "3.9"])
+@nox.session(python=["2.7", "3.6", "3.7", "3.8", "3.9"])
 def unit(session):
     """Run the unit test suite."""
     default(session)
 
 
-@nox.session(python=["2.7", "3.5", "3.6", "3.7", "3.8", "3.9"])
+@nox.session(python=["2.7", "3.6", "3.7", "3.8", "3.9"])
 def unit_grpc_gcp(session):
     """Run the unit test suite with grpcio-gcp installed."""