Add information on the Python version in CONTRIBUTING.md (#170)

Add more information on how to install the correct Python version.

Also update the poetry command to install the dependencies.

CONTRIBUTING.md

@@ -3,7 +3,11 @@
 The best way to make a contribution is to first fork the repository on github, 
 create a new branch with an explicit name for your change, and then create a pull request with your changes.
 
-We use [poetry](https://python-poetry.org/) as our package manager, 
+The project requires Python 3.11. Make sure that you have a compatible version.
+Using [Pyenv](https://github.com/pyenv/pyenv) is an easy way to get the right
+python version.
+
+We use [poetry](https://python-poetry.org/) as our package manager,
 so make sure it is installed and you have run `poetry install` in your directory.
 
 When you add features or change APIs, update `CHANGELOG.md` to track new changes.
@@ -21,7 +25,7 @@ This project is fully typed, so any contribution should provide typing annotatio
 
 To run the tools locally, make sure that you have installed dependencies with dev group:
 ```sh
-poetry install --with=dev --with=test
+poetry install --sync
 ```
 
 Then you can run: