Add instructions for pyyml, jinja2, default Python

Installing `mosh` via Homebrew may change `/usr/local/bin/python` to
Python 2. Adds docs to check and fix this so that `pyyml` and `jinja2`
can be installed.

prepare-vms/README.md

@@ -17,9 +17,10 @@ the Azure CLI, the AWS CLI, or terraform (for OpenStack deployment).
 
 And if you want to generate printable cards:
 
-- [pyyaml](https://pypi.python.org/pypi/PyYAML) (on a Mac: `brew install pyyaml`)
-- [jinja2](https://pypi.python.org/pypi/Jinja2) (on a Mac: `brew install jinja2`)
-
+- [pyyaml](https://pypi.python.org/pypi/PyYAML) `pip install pyyaml`
+- [jinja2](https://pypi.python.org/pypi/Jinja2) `pip install jinja2`
+-
+-(These require Python 3; see below for information on setting Python3 as your default Python on the Mac. If you installed `mosh`, Homebrew may have changed your default Python to Python 2.) 
 ## General Workflow
 
 - fork/clone repo
@@ -256,3 +257,22 @@ If you don't have `wkhtmltopdf` installed, you will get a warning that it is a m
 
   - Don't write to bash history in system() in postprep
   - compose, etc version inconsistent (int vs str)
+
+## Making sure Python3 is your default on the Mac
+
+Check the `/usr/local/bin/python` symlink. It should be pointing to `//usr/local/Cellar/python/3`-something. If it isn't, do the following:
+
+ - Verify that python 3 is installed:
+ -
+   ls -la /usr/local/Cellar/Python
+
+ - You should see one or more versions of Python 3. If you don't, `brew install python` to install it.
+ - Verify that `python` points to Python3.
+ 
+    ls -la /usr/local/bin/python
+
+ - If this points to `/usr/local/Cellar/python@2`, then we'll need to change it.
+ 
+   rm /usr/local/bin/python
+   ln -s /usr/local/Cellar/Python/xxxx /usr/local/bin/python # where xxxx is the most recent Python 3 version you saw above
+