cppcms: use python2

pkgs/development/libraries/cppcms/default.nix

···
-{ lib, stdenv, fetchurl, cmake, pcre, zlib, python, openssl }:
+{ lib, stdenv, fetchurl, cmake, pcre, zlib, python2, openssl }:
 
 stdenv.mkDerivation rec {
   pname = "cppcms";
···
   };
 
   nativeBuildInputs = [ cmake ];
-  buildInputs = [ pcre zlib python openssl ];
+  buildInputs = [ pcre zlib python2 openssl ];
+
+  strictDeps = true;
 
   cmakeFlags = [
     "--no-warn-unused-cli"