Add a time limit to Python version support

Currently, we are supporting Python version until their usage falls
low-enough. This worked well, when we had fewer users and Python's
sliding window moved slower.

Adding a time limit clause helps avoid the situation that happened with
Python 2.7, where support had to be maintained until an undetermined
time at the point of hitting end of life. Since our metrics for
determining usage levels are not particularly robust against being
overrepresentative of certain usage patterns and we're seeing this in
the current usage numbers, this also ensures that the maintainers of
Python's packaging tooling aren't indefinitely subsidising the cost of
operation for organisational users, for organisations that are using
EOL Python versions.

docs/html/development/release-process.rst

@@ -93,7 +93,8 @@ issues by pip's maintainers.
 Python Support Policy
 ---------------------
 
-In general, a given Python version is supported until its usage on PyPI falls below 5%.
+In general, a given Python version is supported until its usage on PyPI falls below 5%
+or 6 months after end of life for the CPython release (whichever happens first).
 This is at the maintainers' discretion, in case extraordinary circumstances arise.
 
 .. _`Feature Flags`: