Update the Python version policy (#18559)

docs/reference/policies/python.md

@@ -2,16 +2,31 @@
 
 ## Python versions
 
-uv supports and is tested against the following Python versions:
+uv has Tier 1 support for the following Python versions:
 
-- 3.8
-- 3.9
 - 3.10
 - 3.11
 - 3.12
 - 3.13
 - 3.14
-- 3.15
+
+As with [platforms](./platforms.md), Tier 1 support can be thought of "guaranteed to work". uv is
+continuously tested against these versions.
+
+uv has Tier 2 support for:
+
+- 3.6
+- 3.7
+- 3.8
+- 3.9
+
+uv is "expected to work" with these versions. uv is tested against these versions, but they have
+reached their [end-of-life](https://devguide.python.org/versions/) and no longer receive security
+fixes. We do not recommend using these versions.
+
+uv also has Tier 2 support for pre-releases of Python 3.15.
+
+uv does not work with Python versions prior to 3.6.
 
 ## Python implementations