Update Mac setup guide (GoogleCloudPlatform#2697)

Update Mac setup guide

Fixes GoogleCloudPlatform#2693

Add instructions for Catalina
Move installation verification after installation of specific versions
Update latest python versions and add commands for each
Add console label to code blocks
Remove username

Author: texasmichelle

MAC_SETUP.md

@@ -10,23 +10,26 @@ test their code.
 
 ## Before you begin
 
-1.  [Optional] Install [homebrew](https://brew.sh/).
+1. Install [homebrew](https://brew.sh/) if you do not already have it.
+
+   **Note:** If you are running Catalina (MacOS 10.15.x), ensure that you have a
+   compatible version of Homebrew (2.1.13 or later). Running `brew update` on
+   Catalina does not always result in a compatible version, so uninstall and
+   reinstall homebrew, if necessary.
 
 ## Installing pyenv and pyenv-virtualenv
 
 1.  Install [pyenv](https://github.com/pyenv/pyenv).
 
-    I (tswast@) use [homebrew](https://brew.sh/) to install it.
-
-    ```
+    ```console
     brew update
     brew install pyenv
     ```
 
 1.  Install the [pyenv-virtualenv](https://github.com/pyenv/pyenv-virtualenv)
     plugin.
 
-    ```
+    ```console
     brew install pyenv-virtualenv
     ```
 
@@ -37,80 +40,85 @@ test their code.
     eval "$(pyenv virtualenv-init -)"
     ```
 
-    Note that this also works with ZSH.
+    **Note:** This also works with ZSH.
 
 1.  Reload your shell.
 
-    ```
+    ```console
     source ~/.bashrc
     ```
 
-1.  After you have installed a python version through pyenv (see below), 
-    verify that you are now using the pyenv Python shim.
-
-    ```
-    $ which python
-    /Users/tswast/.pyenv/shims/python
-    ```
-
 ## Installing multiple Python versions
 
 
 1.  See the available Python versions with
 
-    ```
+    ```console
     pyenv install --list
     ```
 
-    The Python versions are at the top of the long list. If the Python
+    **Note:** The Python versions are at the top of the long list. If the Python
     version you want isn't listed, you may need to upgrade your pyenv with
     homebrew.
 
-    ```
+    ```console
     brew update
     brew upgrade pyenv
     ```
     
-    View the options related to a specific release with
-    
+1.  Install the necessary Python versions with pyenv. Use the latest release
+    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:
+
+    *  2.7.17 (latest 2.7.x release)
+    ```console
+    $ pyenv install 2.7.17
     ```
-    pyenv install 3.x
+    *  3.5.9 (latest 3.5.x release)
+    ```console
+    $ pyenv install 3.5.9
+    ```
+    *  3.6.10 (latest 3.6.x release)
+    ```console
+    $ pyenv install 3.6.10
+    ```
+    *  3.7.6 (latest 3.7.x release)
+    ```console
+    $ pyenv install 3.7.6
     ```
-    
-    
 
-1. Compile the necessary Python versions with pyenv. Use the latest release
-   of the versions you wish to test against.  A list of available versions
-   is available on [python.org](https://www.python.org/doc/versions/)
+1.  After you have installed a python version through pyenv,
+    verify that you are now using the pyenv Python shim.
 
-    As of December 9, 2019 latest Python versions are:
+    ```console
+    $ which python
+    ~/.pyenv/shims/python
+    ```
 
-    *  2.7.17 (latest 2.7.x release)
-    *  3.5.8 (latest 3.5.x release)
-    *  3.6.9 (latest 3.6.x release)
-    *  3.7.5 (latest 3.7.x release)
 
 ## Using pyenv and pyenv-virtualenv to manage your Python versions
 
 1.  Change to the desired source directory.
 
-    ```
+    ```console
     cd ~/src/python-docs-samples
     ```
 
 1.  Create a virtualenv using `pyenv virtualenv`.
 
-    ```
-    pyenv virtualenv 3.6.4 python-docs-samples
+    ```console
+    pyenv virtualenv 3.7.6 python-docs-samples
     ```
 
     This creates a virtualenv folder within `~/.pyenv/versions/`.
 
 1.  Set the local Python version(s) with `pyenv local`
 
-    ```
+    ```console
     # pyenv local [name of virtualenv] [list of python versions to use]
-    pyenv local python-docs-samples 3.6.4 3.7.0 3.5.4 2.7.15
+    pyenv local python-docs-samples 3.6.10 3.7.6 3.5.9 2.7.17
     ```
 
 1.  Now, when you `cd` into the source directory or a subdirectory within it,
@@ -120,4 +128,4 @@ test their code.
 
 1.  Add `.python-version` to your [global gitignore
     file](https://help.github.com/articles/ignoring-files/#create-a-global-gitignore),
-    so it wont be committed into the repository.
+    so it won't be committed into the repository.