[cleanup] cleanup interwiki_graph.py
[cleanup] cleanup interwiki_graph.py

- remove Python 2 code parts
- add deprecated decorator to already deprecated properties
- replace all originPage occurrences by origin
- remove old originPage variable in Subject.__init__
- update doc in Subject.__init__
- initialize found_in attribute only once
- deprecate global variable 'pydotfound'
- remove sanity check for pydot 1.0.3 because the lowest pydot version
  allowed is 1.2

detached from 70ebe6db0980c4cece96356a0b8673a78c916914

Change-Id: I19d8678ec9f9241c9beca90f1cda3af2b62ab86c

wikimedia/pywikibot-core,wikimedia/pywikibot-core