Python interpreter discovery (#50163)

* Python interpreter discovery

* No longer blindly default to only `/usr/bin/python`
* `ansible_python_interpreter` defaults to `auto_legacy`, which will discover the platform Python interpreter on some platforms (but still favor `/usr/bin/python` if present for backward compatibility). Use `auto` to always use the discovered interpreter, append `_silent` to either value to suppress warnings.
* includes new doc utility method `get_versioned_doclink` to generate a major.minor versioned doclink against docs.ansible.com (or some other config-overridden URL)

* docs revisions for python interpreter discovery

(cherry picked from commit 5b53c0012ab7212304c28fdd24cb33fd8ff755c2)

* verify output on some distros, cleanup

docs/docsite/rst/reference_appendices/interpreter_discovery.rst

@@ -0,0 +1,51 @@
+.. _interpreter_discovery:
+
+Interpreter Discovery
+=====================
+
+Most Ansible modules that execute under a POSIX environment require a Python
+interpreter on the target host. Unless configured otherwise, Ansible will
+attempt to discover a suitable Python interpreter on each target host
+the first time a Python module is executed for that host.
+
+To control the discovery behavior:
+
+* for individual hosts and groups, use the ``ansible_python_interpreter`` inventory variable
+* globally, use the ``interpreter_python`` key in the ``[defaults]`` section of ``ansible.cfg``
+
+Use one of the following values:
+
+auto_legacy : (default in 2.8)
+  Detects the target OS platform, distribution, and version, then consults a
+  table listing the correct Python interpreter and path for each
+  platform/distribution/version. If an entry is found, and ``/usr/bin/python`` is absent, uses the discovered interpreter (and path). If an entry
+  is found, and ``/usr/bin/python`` is present, uses ``/usr/bin/python``
+  and issues a warning.
+  This exception provides temporary compatibility with previous versions of
+  Ansible that always defaulted to ``/usr/bin/python``, so if you have
+  installed Python and other dependencies at ``usr/bin/python`` on some hosts,
+  Ansible will find and use them with this setting.
+  If no entry is found, or the listed Python is not present on the
+  target host, searches a list of common Python interpreter
+  paths and uses the first one found; also issues a warning that future
+  installation of another Python interpreter could alter the one chosen.
+
+auto : (future default in 2.12)
+  Detects the target OS platform, distribution, and version, then consults a
+  table listing the correct Python interpreter and path for each
+  platform/distribution/version. If an entry is found, uses the discovered
+  interpreter.
+  If no entry is found, or the listed Python is not present on the
+  target host, searches a list of common Python interpreter
+  paths and uses the first one found; also issues a warning that future
+  installation of another Python interpreter could alter the one chosen.
+
+auto_legacy_silent
+  Same as ``auto_legacy``, but does not issue warnings.
+
+auto_silent
+  Same as ``auto``, but does not issue warnings.
+
+You can still set ``ansible_python_interpreter`` to a specific path at any
+variable level (for example, in host_vars, in vars files, in playbooks, etc.).
+Setting a specific path completely disables automatic interpreter discovery; Ansible always uses the path specified.

docs/docsite/rst/reference_appendices/python_3_support.rst

@@ -39,9 +39,12 @@ command via ``python3``.  For example:
 Using Python 3 on the managed machines with commands and playbooks
 ------------------------------------------------------------------
 
-* Set the ``ansible_python_interpreter`` configuration option to :command:`/usr/bin/python3`. The
-  ``ansible_python_interpreter`` configuration option is usually set as an inventory
-  variable associated with a host or group of hosts:
+* Ansible will automatically detect and use Python 3 on many platforms that ship with it. To explicitly configure a
+  Python 3 interpreter, set the ``ansible_python_interpreter`` inventory variable at a group or host level to the
+  location of a Python 3 interpreter, such as :command:`/usr/bin/python3`. The default interpreter path may also be
+  set in ``ansible.cfg``.
+
+.. seealso:: :ref:`interpreter_discovery` for more information.
 
 .. code-block:: ini