docs: describe the support policy for python versions

Bug: https://github.com/mesonbuild/meson/issues/14322

docs/markdown/Contributing.md

@@ -501,11 +501,58 @@ those are simple.
 - indent 4 spaces, no tabs ever
 - brace always on the same line as if/for/else/function definition
 
-## External dependencies
+## Dependency support policy
 
 The goal of Meson is to be as easily usable as possible. The user
 experience should be "get Python3 and Ninja, run", even on
-Windows. Unfortunately this means that we can't have dependencies on
+Windows.
+
+Additionally, Meson is popularly used in many core infrastructure packages in a
+Unix (and particularly, Linux) userland. This includes:
+- package managers, such as pacman (Arch Linux) and portage (Gentoo)
+- init systems (systemd, openrc, dinit)
+- graphics stacks (xorg, wayland, libdrm, Mesa, gtk)
+
+As such it needs to be able to run early on when bootstrapping a system from
+scratch.
+
+### Python
+
+We will always support all non EOL versions of CPython. Yes, there are people
+out there using and depending on every old version of python. In fact, there
+are people using and depending on systems that had a brand new python at the
+time of release, but with a much longer support cycle than Python itself. We
+need to balance the tradeoff between supporting those systems and being able to
+improve our own codebase and code quality.
+
+Meson will also be *honest* about what versions of python it supports. When a
+version of CPython becomes EOL, it becomes eligible to be removed from our
+support policy. We cannot guarantee continued support forever for software that
+is not supported by its own developers, even if some deprecated LTS systems out
+there still ship it. However, that doesn't mean we will drop support for those
+versions simply because they are old. If we are not using new functionality
+from new python versions, we will continue to mark Meson as compatible with the
+older version -- and test it in CI!
+
+(Note that contrary to popular belief, it is actually easier to test support
+for very old versions of python than it is to drop support for it. We already
+have the CI setup necessary for testing. Upgrading the CI to use newer versions
+of python, on the other hand, represents mildly painful administrative work
+that has to be done.)
+
+So, in order to start requiring a newer version of python, one should check a
+few factors:
+- are the older versions being dropped, already EOL? [Python EOL chart](https://endoflife.date/python)
+- document the new minimum version of corresponding OSes
+- rationalize the benefit of the change in terms of improvements to development
+  and maintenance of Meson. What new language features will be unlocked by the
+  upgrade, that Meson will be able to make good use of? Not every version has
+  new features requiring an upgrade, and not every new feature is so great we
+  need to drop everything to use it
+
+### External dependencies
+
+Unfortunately this also means that we can't have dependencies on
 projects outside of Python's standard library. This applies only to
 core functionality, though. For additional helper programs etc the use
 of external dependencies may be ok. If you feel that you are dealing

docs/markdown/FAQ.md

@@ -333,6 +333,24 @@ that could fulfill these requirements:
 Out of these we have chosen Python because it is the best fit for our
 needs.
 
+## Do you at least support my ancient python install?
+
+Yes! :) We have a relatively sedate version support policy. You can read about
+it in the [Contributing documentation](Contributing.md#python)
+
+We are also willing to support old versions of meson as LTS releases,
+particularly, if it is the final version to support a given python version. If
+you have a use case, please discuss it with us and be willing to help backport
+bug fixes.
+
+- python 3.5: [supported through Meson 0.56.2](Release-notes-for-0.56.0.md#python-35-support-will-be-dropped-in-the-next-release)
+- python 3.6: [supported through Meson 0.61.5](Release-notes-for-0.61.0.md#python-36-support-will-be-dropped-in-the-next-release)
+- python 3.7: currently actively supported by Meson
+
+We encourage projects to support a wide range of Meson versions if they are not
+actually using the latest features anyway. In many, many cases it is quite
+practical to support e.g. Meson 0.61.
+
 ## But I really want a version of Meson that doesn't use python!
 
 Ecosystem diversity is good. We encourage interested users to write this