bpython crashes w/python3

[karlredman]

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Hi, I have a fix for this that I can do a PR for in a bit (still testing).

The issue is that bpython crashes after importing calculon on python 3.5.3 and above after having started a 'display' already for calculon. This seems related to a difference in python v2.7 vs v3+ object instantiation rules.

Note that I use pyenv but the issue occurs at system level as well -so it's not environmental. here's the console side error:

$ bpython
bpython version 0.17.1 on top of Python 3.6.6 /home/karl/.pyenv/versions/3.6.6/bin/python3.6
>>> import calculon.load
Connected to calculon
>>> 1
Traceback (most recent call last):
  File "/home/karl/.pyenv/versions/3.6.6/bin/bpython", line 11, in <module>
    sys.exit(main())
  File "/home/karl/.pyenv/versions/3.6.6/lib/python3.6/site-packages/bpython/curtsies.py", line 191, in main
    exit_value = repl.mainloop(True, paste)
  File "/home/karl/.pyenv/versions/3.6.6/lib/python3.6/site-packages/bpython/curtsies.py", line 126, in mainloop
    self.process_event_and_paint(e)
  File "/home/karl/.pyenv/versions/3.6.6/lib/python3.6/site-packages/bpython/curtsies.py", line 90, in process_event_and_paint
    self.process_event(e)
  File "/home/karl/.pyenv/versions/3.6.6/lib/python3.6/site-packages/bpython/curtsiesfrontend/repl.py", line 603, in process_event
    return self.process_key_event(e)
  File "/home/karl/.pyenv/versions/3.6.6/lib/python3.6/site-packages/bpython/curtsiesfrontend/repl.py", line 732, in process_key_event
    self.on_enter()
  File "/home/karl/.pyenv/versions/3.6.6/lib/python3.6/site-packages/bpython/curtsiesfrontend/repl.py", line 817, in on_enter
    self.push(self.current_line, insert_into_history=insert_into_history)
  File "/home/karl/.pyenv/versions/3.6.6/lib/python3.6/site-packages/bpython/curtsiesfrontend/repl.py", line 1087, in push
    self.run_code_and_maybe_finish()
  File "/home/karl/.pyenv/versions/3.6.6/lib/python3.6/site-packages/bpython/curtsiesfrontend/repl.py", line 1090, in run_code_and_maybe_finish
    r = self.coderunner.run_code(for_code=for_code)
  File "/home/karl/.pyenv/versions/3.6.6/lib/python3.6/site-packages/bpython/curtsiesfrontend/coderunner.py", line 138, in run_code
    request = self.code_context.switch()
  File "/home/karl/.pyenv/versions/3.6.6/lib/python3.6/site-packages/bpython/curtsiesfrontend/coderunner.py", line 180, in _blocking_run_code
    unfinished = self.interp.runsource(self.source)
  File "/home/karl/.local/lib/python3.6/site-packages/calculon_calc-0.2-py3.6.egg/calculon/repl.py", line 163, in runsource
    self.locals['disp'] = disp
NameError: name 'disp' is not defined