mongodb: 4.0.12 -> 4.2.8

Not strictly an upgrade, but adds a new mongodb-4_2 target with the
current mongodb from that branch.

Use matching client and server versions in mongodb tests- tests were
using the mongo 3.4 client to connect, and this finally doesn't work
with server 4.2.

Per reviewer suggestion, adding myself as cheetah3 maintainer.

Additionally, reestore comments describing the purpose of the
build-dependencies patch

pkgs/development/python-modules/cheetah3/default.nix

@@ -0,0 +1,20 @@
+{ lib, buildPythonPackage, fetchPypi, stdenv }:
+
+buildPythonPackage rec {
+  pname = "Cheetah3";
+  version = "3.2.5";
+
+  src = fetchPypi {
+    inherit pname version;
+    sha256 = "ececc9ca7c58b9a86ce71eb95594c4619949e2a058d2a1af74c7ae8222515eb1";
+  };
+
+  doCheck = false; # Circular dependency
+
+  meta = {
+    homepage = "http://www.cheetahtemplate.org/";
+    description = "A template engine and code generation tool";
+    license = lib.licenses.mit;
+    maintainers = with stdenv.lib.maintainers; [ pjjw ];
+  };
+}