Added information about currently supported Python versions (#13029)

Added information to the Upgrading to 2.0 doc and the installation doc
to show the currently supported Python versions.

docs/apache-airflow/installation.rst

@@ -68,6 +68,9 @@ and python versions in the URL.
     # For example: https://raw.githubusercontent.com/apache/airflow/constraints-1.10.14/constraints-3.6.txt
     pip install "apache-airflow==${AIRFLOW_VERSION}" --constraint "${CONSTRAINT_URL}"
 
+    Please note that with respect to Python 3 support, Airflow 1.10.14 has been
+    tested with Python 3.6, 3.7, and 3.8, but does not yet support Python 3.9.
+
 2. Installing with extras (for example postgres, google)
 
 .. note::

docs/apache-airflow/upgrading-to-2.rst

@@ -27,7 +27,9 @@ users to migrate from Airflow 1.10.x to Airflow 2.0.
 Step 1: Upgrade to Python 3
 '''''''''''''''''''''''''''
 
-Airflow 1.10 will be the last release series to support Python 2. Airflow 2.0.0 will require Python 3.6+.
+Airflow 1.10 will be the last release series to support Python 2. Airflow 2.0.0 will
+require Python 3.6+ and at this point in time has been tested with Python versions 3.6, 3.7,
+and 3.8, but does not yet support Python 3.9.
 
 If you have a specific task that still requires Python 2 then you can use the :class:`~airflow.operators.python.PythonVirtualenvOperator` or the ``KubernetesPodOperator`` for this.