ActiveDoc: make Python 3 the default version

It's already default in various contexts, but sometimes there is
nothing defining what the Python version, which makes it default to 2.
This has broken some documents in the past. It's high time to change
this default.

app/server/lib/ActiveDoc.ts

@@ -2830,7 +2830,7 @@ export class ActiveDoc extends EventEmitter {
 
   private async _makeEngine(): Promise<ISandbox> {
     // Figure out what kind of engine we need for this document.
-    let preferredPythonVersion: '2' | '3' = process.env.PYTHON_VERSION === '3' ? '3' : '2';
+    let preferredPythonVersion: '2' | '3' = process.env.PYTHON_VERSION === '2' ? '2' : '3';
 
     // Careful, migrations may not have run on this document and it may not have a
     // documentSettings column.  Failures are treated as lack of an engine preference.