Document Python support policy

Author: hugovk

docs/html/development/release-process.rst

@@ -65,7 +65,7 @@ their merits.
   ``pip._internal.utils.deprecation.deprecated``. The function is not a part of
   pip's public API.
 
-Python 2 support
+Python 2 Support
 ----------------
 
 pip will continue to ensure that it runs on Python 2.7 after the CPython 2.7
@@ -79,6 +79,12 @@ only bugs will be considered, and merged (subject to normal review processes).
 Note that there may be delays due to the lack of developer resources for
 reviewing such pull requests.
 
+Python Support Policy
+---------------------
+
+In general, a given Python version is supported until its usage on PyPI falls below 5%.
+This is at the maintainers' discretion, in case extraordinary circumstances arise.
+
 .. _`Feature Flags`:
 
 Feature Flags

news/8927.removal

@@ -0,0 +1 @@
+Document that Python versions are generally supported until PyPI usage falls below 5%.