[emr] document why the bootstrap targets Python 3.11

EMR 7.x officially ships only Python 3.9 and 3.11; its default python3
is 3.9, too old for Hail. Explain in the EMR_PYSPARK_PYTHON constant and
the user docs why the bootstrap installs into 3.11 and how to update the
target if a future EMR release ships a newer supported Python.

Author: Brandon

hail/python/hail/docs/cloud/amazon_web_services.rst

@@ -50,8 +50,13 @@ Choosing an EMR release
 Hail requires Spark 3.5.x. The default release, ``emr-7.3.0``, provides Spark 3.5.3, which matches
 the Spark version Hail is built against. If you pass a ``--release-label`` that ships a different
 Spark minor version, ``hailctl emr start`` refuses to start the cluster; an unrecognized label
-produces a warning. Hail requires Python 3.10 or newer; the bootstrap installs Hail into Python
-3.11, which ships with EMR releases 7.1.0 and newer, so use ``emr-7.1.0`` or later.
+produces a warning.
+
+Hail also requires a supported version of Python. The EMR 7.x application stack officially ships
+only Python 3.9 and 3.11, and its default ``python3`` is 3.9, which is too old for Hail. The
+bootstrap therefore installs Hail into Python 3.11 (available on EMR releases 7.1.0 and newer) and
+points Spark's driver and executors at it, so use ``emr-7.1.0`` or later. If a future EMR release
+ships a newer default Python that Hail supports, ``hailctl emr`` will be updated to target it.
 
 Advanced cluster options
 ------------------------

hail/python/hailtop/hailctl/emr/start.py

@@ -17,9 +17,12 @@
 DEFAULT_EMR_RELEASE = 'emr-7.3.0'
 HAIL_REQUIRED_SPARK_MINOR = '3.5'
 HAIL_JAR_PATH = '/usr/lib/hail/hail-all-spark.jar'
-# EMR 7.x's system python3 is 3.9, but Hail requires Python >= 3.10. The
-# bootstrap installs Hail into Python 3.11 (shipped with EMR 7.1+); PySpark
-# (driver and executors) must use the same interpreter.
+# EMR 7.x's system python3 is 3.9, but Hail requires Python >= 3.10. The EMR 7.x
+# application stack officially ships only Python 3.9 and 3.11 (see the release's
+# component versions); 3.12 is not provided by default. So the bootstrap installs
+# Hail into Python 3.11, and PySpark (driver and executors) must use that same
+# interpreter. If a future EMR release ships a newer default Python that Hail
+# supports, update this single constant (and the bootstrap's dnf line).
 EMR_PYSPARK_PYTHON = '/usr/bin/python3.11'