readme: explain venv, then pip

README.md

@@ -32,13 +32,9 @@ For more details on each of the stages, see the [`docs`](docs/) directory.
 
 ## Usage
 ### Dependencies
-Python 3 is required.
+Python 3.9 or newer is required. We primarily test on Python 3.11.
 
 #### Python
-With `pip` installed, run the following in the root directory:
-
-    pip install -e .
-
 To avoid any conflicts with local packages, we recommend using a virtual
 environment. In the root directory:
 
@@ -47,7 +43,13 @@ environment. In the root directory:
 
 See [virtualenv docs](https://virtualenv.pypa.io/en/latest/) for more details.
 
-Alternatively, if you are using python to manage virtual environments using the `venv` module, then the first line above can be replaced by `python -m venv venv` (where the second `venv` is the virtual environment name).
+Alternatively, if you are using python to manage virtual environments using the
+`venv` module, then the first line above can be replaced by `python -m venv
+venv` (where the second `venv` is the virtual environment name).
+
+With `pip` installed, run the following in the root directory:
+
+    pip install -e .
 
 *(An alternate `pyproject.toml` file is provided for building with
 [Poetry](https://python-poetry.org/). To use, rename `pyproject-poetry.toml` to