docs: Add python version support to backcompat (#3115)

Co-authored-by: Francesco Bruzzesi <[email protected]>

Author: dangotbanned

docs/backcompat.md

@@ -98,6 +98,7 @@ Here are exceptions to our backwards compatibility policy:
 - We may consider making some type hints more precise.
 - Anything labelled "unstable".
 - We may sometimes need to bump the minimum versions of supported backends.
+- We will drop support for old python versions, roughly in sync with [their end of life](https://devguide.python.org/versions/#supported-versions)
 
 In general, decision are driven by use-cases, and we conduct a search of public GitHub repositories
 before making any change.