remove python 3.5 config (#3022)

* remove python 3.5 config

* Remove from noxfile

* Update authoring guide

Author: leahecole

.kokoro/python3.5/common.cfg

@@ -19,8 +19,8 @@ No matter what, all samples must:
 1. Be either a web application or a runnable console application.
 1. Have a `requirements.txt` containing all of its third-party dependencies. All
 requirements must be pinned.
-1. Work in Python 2.7, 3.5, 3.6, and 3.7. App Engine Standard is exempt as it
-   only supports 2.7. Our default version is currently Python 3.5.
+1. Work in Python 2.7, 3.6, and 3.7. App Engine Standard is exempt as it
+   only supports 2.7. Our default version is currently Python 3.6.
 1. Have tests.
 
 ## Style & linting

.kokoro/python3.5/continuous.cfg

@@ -208,7 +208,7 @@ def py2(session, sample):
     _session_tests(session, sample)
 
 
-@nox.session(python=["3.5", "3.6", "3.7"])
+@nox.session(python=["3.6", "3.7"])
 @nox.parametrize("sample", NON_GAE_STANDARD_SAMPLES_PY3)
 def py3(session, sample):
     """Runs py.test for a sample using Python 3.x"""