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

docs/source/_ext/fetch_md.py

@@ -17,12 +17,6 @@ from docutils.nodes import SparseNodeVisitor
 from docutils.parsers.rst import Directive
 from utils import md_to_docutils, get_link_key
 
-# Python2 compatibility
-try:
-    FileNotFoundError
-except NameError:
-    FileNotFoundError = IOError
-
 
 class Relinker(SparseNodeVisitor):