Correctly document min python version

And stop using `python3`

Author: hyanwong

docs/installation.rst

@@ -4,7 +4,7 @@
 Installation
 ############
 
-Python 3.7 or newer is required for ``tsinfer``. Any Unix-like platform should
+Python 3.9 or newer is required for ``tsinfer``. Any Unix-like platform should
 work (``tsinfer`` is tested on Linux, OS X, and Windows).
 
 ***************
@@ -26,14 +26,14 @@ Installing from source
 
 It is also possible to install from source via ``pip`` (although see the issues below):
 
-    $ python3 -m pip install tsinfer --user
+    $ python -m pip install tsinfer --user
 
 which will install ``tsinfer`` to the Python installation corresponding to your
-``python3`` executable. All requirements should be installed automatically.
+``python`` executable. All requirements should be installed automatically.
 
 To run the command line interface to ``tsinfer`` you can then use::
 
-    $ python3 -m tsinfer --help
+    $ python -m tsinfer --help
 
 
 If your ``PATH`` is set up to point at the corresponding ``bin`` directory
@@ -44,9 +44,9 @@ you can also use the ``tsinfer`` executable directly::
 You may wish to install into a virtual environment
 first using `venv <https://docs.python.org/3/library/venv.html>`_::
 
-    $ python3 -m venv tsinfer-venv
+    $ python -m venv tsinfer-venv
     $ source tsinfer-venv/bin/activate
-    (tsinfer-venv) $ python3 -m pip install tsinfer
+    (tsinfer-venv) $ python -m pip install tsinfer
     (tsinfer-venv) $ tsinfer --help
 
 ****************