Update python versions (GoogleCloudPlatform#4655)

Author: David Cavazos

MAC_SETUP.md

@@ -70,27 +70,27 @@ test their code.
     of the versions you wish to test against.  A list of available versions
     is available on [python.org](https://www.python.org/doc/versions/)
 
-    As of January 8, 2020, the latest Python versions are:
+    As of September 9, 2020, the latest Python versions are:
 
-    *  2.7.17 (latest 2.7.x release)
+    *  2.7.18 (latest 2.7.x release)
     ```console
-    $ pyenv install 2.7.17
+    $ pyenv install 2.7.18
     ```
     *  3.5.9 (latest 3.5.x release)
     ```console
     $ pyenv install 3.5.9
     ```
-    *  3.6.10 (latest 3.6.x release)
+    *  3.6.11 (latest 3.6.x release)
     ```console
-    $ pyenv install 3.6.10
+    $ pyenv install 3.6.11
     ```
-    *  3.7.6 (latest 3.7.x release)
+    *  3.7.8 (latest 3.7.x release)
     ```console
-    $ pyenv install 3.7.6
+    $ pyenv install 3.7.8
     ```
-    *  3.8.1 (latest 3.8.x release)
+    *  3.8.5 (latest 3.8.x release)
     ```console
-    $ pyenv install 3.8.1
+    $ pyenv install 3.8.5
     ```
 
     > ℹ️ *Note*: If you are getting errors installing a python version,
@@ -113,11 +113,11 @@ test their code.
 Pyenv allows you to configure the priority order for your python installs.
 
 ```
-pyenv global 3.8.1 3.7.6 3.6.10 3.5.9 2.7.17
+pyenv global 3.8.5 3.7.8 3.6.11 3.5.9 2.7.18
 ```
 
-This will make python and python3 point to Python 3.8.1. python2 will use
-2.7.17. You can also further specify versions, such as python3.6 to use that
+This will make python and python3 point to Python 3.8.5. python2 will use
+2.7.18. You can also further specify versions, such as python3.6 to use that
 version.
 
 ## Python virtual environments
@@ -153,10 +153,10 @@ created virtual environment. You can install this by running
     cd ~/src/python-docs-samples	
     ```	
 
-1.  Create a virtualenv for python 3.8.1 using `pyenv virtualenv`.	
+1.  Create a virtualenv for python 3.8.5 using `pyenv virtualenv`.	
 
     ```console	
-    pyenv virtualenv 3.8.1 python-docs-samples	
+    pyenv virtualenv 3.8.5 python-docs-samples	
     ```	
 
     This creates a virtualenv folder within `~/.pyenv/versions/`.	
@@ -165,7 +165,7 @@ created virtual environment. You can install this by running
 
     ```console	
     # pyenv local [name of virtualenv] [list of python versions to use]	
-    pyenv local python-docs-samples 3.8.1 3.7.6 3.6.10 3.5.9 2.7.17	
+    pyenv local python-docs-samples 3.8.5 3.7.8 3.6.11 3.5.9 2.7.18	
     ```	
 
 1.  Now, when you `cd` into the source directory or a subdirectory within it,