Rename Dockerfile && fix typo

templates/tools/dockerfile/test/python_stretch_x64/Dockerfile.template

@@ -1,6 +1,6 @@
 %YAML 1.2
 --- |
-  # Copyright 2016 gRPC authors.
+  # Copyright 2018 gRPC authors.
   #
   # Licensed under the Apache License, Version 2.0 (the "License");
   # you may not use this file except in compliance with the License.
@@ -19,7 +19,7 @@
   <%include file="../../apt_get_basic.include"/>
   <%include file="../../gcp_api_libraries.include"/>
   
-  # Install Python Versions from Debain 'testing' repository
+  # Install Python Versions from Debian 'testing' repository
   RUN echo 'deb http://ftp.de.debian.org/debian testing main' >> /etc/apt/sources.list
   RUN echo 'APT::Default-Release "stable";' | tee -a /etc/apt/apt.conf.d/00local
   RUN apt-get update && apt-get -t testing install -y python2.7 python3.5 python3.6 python3.7 python-all-dev python3-all-dev

tools/dockerfile/test/python_stretch_x64/Dockerfile

@@ -1,4 +1,4 @@
-# Copyright 2016 gRPC authors.
+# Copyright 2018 gRPC authors.
 #
 # Licensed under the Apache License, Version 2.0 (the "License");
 # you may not use this file except in compliance with the License.
@@ -54,7 +54,7 @@ RUN apt-get update && apt-get install -y python-pip && apt-get clean
 RUN pip install --upgrade google-api-python-client oauth2client
 
 
-# Install Python Versions from Debain 'testing' repository
+# Install Python Versions from Debian 'testing' repository
 RUN echo 'deb http://ftp.de.debian.org/debian testing main' >> /etc/apt/sources.list
 RUN echo 'APT::Default-Release "stable";' | tee -a /etc/apt/apt.conf.d/00local
 RUN apt-get update && apt-get -t testing install -y python2.7 python3.5 python3.6 python3.7 python-all-dev python3-all-dev

tools/run_tests/run_tests.py

@@ -760,7 +760,7 @@ class PythonLanguage(object):
 
     def python_manager_name(self):
         if self.args.compiler in ['python3.5', 'python3.6', 'python3.7']:
-            return 'testing'
+            return 'stretch'
         elif self.args.compiler == 'python_alpine':
             return 'alpine'
         else: