Update docs to use Python 3.9 (#2981)

docs/developer_setup.md

@@ -2,7 +2,7 @@
 
 ## Set up the Python virtual environment
 
-First, create a Python virtual environment with Python version >= 3.8 and activate it.
+First, create a Python virtual environment with Python version >= 3.9 and activate it.
 
 Using [Virtualenv](https://docs.python.org/3/library/venv.html#creating-virtual-environments):
 
@@ -18,7 +18,7 @@ Using [Anaconda](https://conda.io/projects/conda/en/latest/user-guide/tasks/mana
 
     # Create a virtual environment.
     # Only run this the first time.
-    conda create -n crfm-helm python=3.8 pip
+    conda create -n crfm-helm python=3.9 pip
 
     # Activate the virtual environment.
     conda activate crfm-helm

docs/installation.md

@@ -2,15 +2,15 @@
 
 ## Create a virtual environment
 
-It is recommended to install HELM into a virtual environment with Python version 3.8 to avoid dependency conflicts. HELM requires Python version 3.8. To create, a Python virtual environment with Python version >= 3.8 and activate it, follow the instructions below.
+It is recommended to install HELM into a virtual environment with Python version >=3.9 to avoid dependency conflicts. HELM requires Python >=3.9. To create, a Python virtual environment and activate it, follow the instructions below.
 
 Using [Virtualenv](https://docs.python.org/3/library/venv.html#creating-virtual-environments):
 
 ```
 # Create a virtual environment.
 # Only run this the first time.
 python3 -m pip install virtualenv
-python3 -m virtualenv -p python3.8 helm-venv
+python3 -m virtualenv -p python3.9 helm-venv
 
 # Activate the virtual environment.
 source helm-venv/bin/activate
@@ -21,7 +21,7 @@ Using [Anaconda](https://conda.io/projects/conda/en/latest/user-guide/tasks/mana
 ```
 # Create a virtual environment.
 # Only run this the first time.
-conda create -n crfm-helm python=3.8 pip
+conda create -n crfm-helm python=3.9 pip
 
 # Activate the virtual environment.
 conda activate crfm-helm