CONTRIBUTING: Python3 is supportet

Explicitly state that Python3 is supported and required.

Since aptly `v1.5.1` (with commit 035d531)
the tests are ported to Python3.

With a687df2 the system
tests are run with `python3` per default.

With f4a152a (after `v1.5.0` aptly tag)
the tests run against Python 3.11.

CONTRIBUTING.md

@@ -74,7 +74,7 @@ This section describes local setup to start contributing to aptly source.
 
 ### Go & Python
 
-You would need `Go` (latest version is recommended) and `Python` 2.7.x (3.x is not supported yet).
+You would need `Go` (latest version is recommended) and `Python` 3.9 (or newer, the CI currently tests against 3.11).
 
 If you're new to Go, follow [getting started guide](https://golang.org/doc/install) to install it and perform
 initial setup. With Go 1.8+, default `$GOPATH` is `$HOME/go`, so rest of this document assumes that.