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/ui/reconstruction.py

@@ -17,12 +17,6 @@ from meshroom.core.node import Node, CompatibilityNode, Status, Position
 from meshroom.ui.graph import UIGraph
 from meshroom.ui.utils import makeProperty
 
-# Python2 compatibility
-try:
-    FileNotFoundError
-except NameError:
-    FileNotFoundError = IOError
-
 
 class Message(QObject):
     """ Simple structure wrapping a high-level message. """
@@ -623,9 +617,8 @@ class Reconstruction(UIGraph):
         sfmFile = panoramaInit.attribute('outSfMData').value
         if not os.path.exists(sfmFile):
             return QVector3D(0.0, 0.0, 0.0)
-        import io  # use io.open for Python2/3 compatibility (allow to specify encoding + errors handling)
         # skip decoding errors to avoid potential exceptions due to non utf-8 characters in images metadata
-        with io.open(sfmFile, 'r', encoding='utf-8', errors='ignore') as f:
+        with open(sfmFile, 'r', encoding='utf-8', errors='ignore') as f:
             data = json.load(f)
 
         intrinsics = data.get('intrinsics', [])