Add notes to changelog and docs for installing on Python < 3.8

Author: mnaberez

CHANGES.rst

@@ -16,6 +16,14 @@
 - On Python 3.8 and later, ``setuptools`` is no longer a runtime
   dependency.  Patch by Ofek Lev.
 
+- On Python versions before 3.8, ``setuptools`` is still a runtime
+  dependency (for ``pkg_resources``) but it is no longer declared in
+  ``setup.py`` as such.  This is because adding a conditional dependency
+  with an environment marker (``setuptools; python_version < '3.8'``)
+  breaks installation in some scenarios, e.g. ``setup.py install`` or
+  older versions of ``pip``.  Ensure that ``setuptools`` in installed
+  if using Python before 3.8.
+
 - ``supervisorctl`` now reads extra files included via the ``[include]``
   section in ``supervisord.conf`` like ``supervisord`` does.  This allows
   the ``[supervisorctl]`` section or ``[ctlplugin:x]`` sections to be in

docs/installing.rst

@@ -22,6 +22,12 @@ to be the root user to install Supervisor successfully using
 
 You can also install supervisor in a virtualenv via ``pip``.
 
+.. note::
+
+   If installing on a Python version before 3.8, first ensure that the
+   ``setuptools`` package is installed because it is a runtime
+   dependency of Supervisor.
+
 Internet-Installing Without Pip
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
@@ -39,6 +45,12 @@ finally install Supervisor itself.
    need to be the root user to successfully invoke ``python
    setup.py install``.
 
+.. note::
+
+   The ``setuptools`` package is required to run ``python setup.py install``.
+   On Python versions before 3.8, ``setuptools`` is also a runtime
+   dependency of Supervisor.
+
 Installing To A System Without Internet Access
 ----------------------------------------------
 
@@ -63,6 +75,12 @@ Finally, run supervisor's ``python setup.py install``.
    need to be the root user to invoke ``python setup.py install``
    successfully for each package.
 
+.. note::
+
+   The ``setuptools`` package is required to run ``python setup.py install``.
+   On Python versions before 3.8, ``setuptools`` is also a runtime
+   dependency of Supervisor.
+
 Installing a Distribution Package
 ---------------------------------