package: add python to nativeBuildInputs

Commit c21d11ac0 "docs: replace sed invocation with an mdbook
preprocessor for @docroot@" added a direct build dependency on Python.
This has been accidentally working so far because Python is already a
*transitive* dependency of Lix's derivation.

Change-Id: I32d6b4f2665dbbfad7014613457dd58aa4ec73da

package.nix

@@ -34,6 +34,7 @@
   ninja,
   openssl,
   pkg-config,
+  python3,
   rapidcheck,
   sqlite,
   toml11,
@@ -153,6 +154,7 @@ in stdenv.mkDerivation (finalAttrs: {
   nativeBuildInputs = [
     bison
     flex
+    python3
   ] ++ [
     (lib.getBin lowdown)
     mdbook