Update python requirement (rust-lang#1182)

Due to the reliance on `cbmc-viewer==2.11` this needs to be python >=3.8 and pip is also required

Author: JustusAdam

docs/src/install-guide.md

@@ -17,7 +17,7 @@ Currently, only two platforms are supported:
 
 The following must already be installed:
 
-* **Python version 3.6 or greater.**
+* **Python version 3.8 or greater** and the package installer pip.
 * Rust installed via `rustup`
 * `ctags` is required for Kani's `--visualize` option to work correctly.