Update Python Support Policy

docs/html/development/release-process.rst

@@ -93,8 +93,14 @@ issues by pip's maintainers.
 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.
+pip supports [CPython versions that are not
+end-of-life](https://devguide.python.org/versions/). Older versions of CPython may be
+supported at the discretion of pip maintainers (based on criteria such as download
+statistics on PyPI, Python versions supported by the vendored dependencies and
+maintenance burden).
+
+pip maintainers accept pull requests to support other Python implementations, but the
+pip CI does not test for compatibility with them.
 
 .. _`Feature Flags`: