Deprecate Python 3.9 support (#14543)

This commit deprecates Python 3.9 support. During the deprecation window
we will continue to fully support Python 3.9 and test it in CI, but it
will emit a warning that we will be removing support for 3.9 in the future
to inform users to upgrade to Python 3.10 or newer. The removal will occur
during the Qiskit 2.3.0 release as it is the first release scheduled to
occur after the upstream Python EoL date in October 2025. [1]

On the development side once we create the stable/2.2 branch on the
2.2.0rc1 release, then we can drop Python 3.9 support and raise the
minimum Python version to 3.10 (including in CI).

[1] https://devguide.python.org/versions/

Author: mtreinish

qiskit/__init__.py

@@ -44,6 +44,14 @@
             " See https://qisk.it/packaging-1-0 for more detail."
         )
 
+if sys.version_info < (3, 10):
+    warnings.warn(
+        "Using Qiskit with Python 3.9 is deprecated as of the 2.1.0 release. "
+        "Support for running Qiskit with Python 3.9 will be removed in the "
+        "2.3.0 release, which coincides with when Python 3.9 goes end of life.",
+        DeprecationWarning,
+    )
+
 from . import _accelerate
 import qiskit._numpy_compat
 

releasenotes/notes/2.1/deprecate-3.9-b0c1d363a38cafbd.yaml

@@ -0,0 +1,10 @@
+---
+deprecations:
+  - |
+    Support for running Qiskit with Python 3.9 has been deprecated and will
+    be removed in the Qiskit 2.3.0 release. The 2.3.0 is the first release after
+    Python 3.9 goes end of life and is no longer supported. [1] This means that
+    starting in the 2.3.0 release you will need to upgrade the Python version
+    you're using to Python 3.9 or above.
+
+    [1] https://devguide.python.org/versions/