Remove workarounds and backports for Python 2 support

In particular:
- In common/, remove the backport of weakref.WeakMethod
- In docs/ and ui/, use the standard FileNotFoundError class
- Use built-in open() instead of io.open(), and stop importing io
- In core/stats.py, use the standard implementation of xml.etree instead
of the C one

meshroom/core/taskManager.py

@@ -348,7 +348,7 @@ class TaskManager(BaseObject):
                 raise ValueError("Argument 'context' must be: 'COMPUTATION' or 'SUBMITTING'")
 
         if len(ready) + len(computed) != len(toNodes):
-            del toNodes[:]  # for python 2 compatibility, else use: toNodes.clear()
+            toNodes.clear()
             toNodes.extend(ready)
             return False