Change how we specify Python versions for testing (Qiskit#1207)

We currently test everything with Python 3.11, but this is a problem as
not all writers have it available on their system. We also want to make
sure our docs work with as many versions of Python as possible.

This PR allows writers to test their docs with any version of Python,
and sets CI to test with Python 3.8 (the minimum version that Qiskit
supports).

.github/actions/set-up-notebook-testing/action.yml

@@ -28,7 +28,7 @@ runs:
   steps:
     - uses: actions/setup-python@v4
       with:
-        python-version: "3.11"
+        python-version: "3.8"
         cache: "pip"
 
     - name: Install Python packages

tox.ini

@@ -1,6 +1,6 @@
 [tox]
 min_version = 4.0
-env_list = py311
+env_list = py3
 skipsdist = true
 
 [testenv]