README.md - people should use exactly 3.13, update instructions about pyenv

Author: jarofgreen

README.md

@@ -24,13 +24,22 @@ Subsequent instructions assume that your current working directory is `os4d`, un
 
 #### Create and activate a Python virtual environment
 
-The following instructions assume you have [Python 3.10](https://www.python.org/downloads/) or newer installed on your machine.
+The following instructions assume you have [Python 3.13](https://www.python.org/downloads/) installed on your machine, as we need to use the same version of python the docs are built with.
 
-You can use either [pyenv-virtualenv](https://github.com/pyenv/pyenv-virtualenv), `python3-venv` or a Python environment manager of your choice:
+If you already have 3.13 you can use `python3-venv` or a Python environment manager of your choice.
+
+If not we recommend pyenv.
 
 ##### pyenv
 
 1. Install [pyenv](https://github.com/pyenv/pyenv) and the [pyenv-virtualenv](https://github.com/pyenv/pyenv-virtualenv) plugin. The [pyenv installer](https://github.com/pyenv/pyenv-installer) is recommended.
+1. Set up Python 3.13 using pyenv:
+
+   ```bash
+   pyenv install 3.13
+   pyenv local 3.13
+   ```
+
 1. Create a virtual environment:
 
    ```bash
@@ -188,4 +197,4 @@ Read the Docs automatically creates and builds [pull request previews](https://d
 
 #### Credentials
 
-You can find credentials for Read the Docs in the Open Data Services password database.
+You can find credentials for Read the Docs in the Open Data Services password database.