chore: remove 2.7 from python showcase tests

It sounds like 2.7 is no longer supported, and it's currently breaking [tests](https://app.circleci.com/pipelines/github/googleapis/gapic-generator/255/workflows/b2063a3b-2ad2-4641-8952-0bc16673a242/jobs/47732/steps) for [this unrelated PR](#3190).

showcase/python/nox.py

@@ -24,7 +24,7 @@ def default(session):
 
 
 @nox.session
-@nox.parametrize('py', ['2.7', '3.5', '3.6', '3.7'])
+@nox.parametrize('py', ['3.5', '3.6', '3.7'])
 def showcase(session, py):
     """Run the showcase integration test suite."""
 
@@ -48,7 +48,7 @@ def showcase(session, py):
 
 
 @nox.session
-@nox.parametrize('py', ['2.7', '3.5', '3.6', '3.7'])
+@nox.parametrize('py', ['3.5', '3.6', '3.7'])
 def unit(session, py):
     """Run the unit test suite."""