Mathics3

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* Adjust for Mathics3 core 6.0.0 API

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* Adjust for Mathics3 core 5.0.0 API
* Fixed autobrace and rewritten with Condition from prompt_toolkit.filter
* Add $GroupAutoComplete setting (default True) to disable completing closer group symbol. Feature provided by DUO Labs and Alessandro Piras.
* Remove bindings for the right brackets
* For prompt-toolkit and GNU Readline users, we allow a user input binding file in CONFIGDIR/inputrc (e.g. ~/.config/mathicsscript/inputrc).
You can set the location this file via environment variable MATHICS_INPUTRC
* Handle version-getting timeout better
* Python 3.6 tolerance
* Create code of conduct
* Some code linting

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* Handle images via PNG and matplotlib
* Packaging: include matplotlib in full install
* Customize Asymptote configuration and add a psviewer that removes eps trash
* Bump minimum Mathics core version to 4.0.0
* Bug fix: Check if `self` has `session` attribute in `asy.__del__`

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* Packaging issues: getting setttings.m file into distribution and other missing files
* More pervasive handling of import errors

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* 3D Graphics is now handled if Asymptote is installed.
* ``Settings`$UseMatplotlib`` and ``Settings`$UseAsymptote`` were added. The can disable the use of matplotlib and Asymptote when those are available.

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Fix packaging issues and be more tolerant matplotlib errors.