README.md: Define Python 3.9 as the minimum minor version

3.8 is going to be EOL in a month, so let's just declare support for everything that's still alive

Author: C0rn3j

README.md

@@ -37,7 +37,7 @@ Donation links for kozec, who is the original developer, can be found on the [ol
 ## Building the package by yourself
 
 ### Dependencies
-  - Python 3
+  - Python 3.9+
   - GTK 3.22+
   - [PyGObject](https://live.gnome.org/PyGObject)
   - [python-gi-cairo](https://packages.debian.org/sid/python-gi-cairo) and [gir1.2-rsvg-2.0](https://packages.debian.org/sid/gir1.2-rsvg-2.0) on Debian-based distributions (included in PyGObject elsewhere)