Boolean.py

----------------

* API changes

* Drop support for Python 2.
* Test on Python 3.10
* Make Expression.sort_order an instance attributes and not a class attribute

* Misc.

* Correct licensing documentation
* Improve docstringf and apply minor refactorings
* Adopt black code style and isort for imports
* Drop Travis and use GitHub actions for CI

----------------

* API changes

* Add support for evaluation of boolean expression.
Thank you to Lars van Gemerden gemerden

* Bug fixes

* Fix parsing of tokens that have a number as the first character.
Thank you to Jeff Cohen jcohen28
* Restore proper Python 2 compatibility.
Thank you to Benjy Weinberger benjyw

* Improve documentation

* Add pointers to Linux distro packages.
Thank you to Max Mehl mxmehl and Carmen Bianca Bakker carmenbianca
* Fix typo.
Thank you to Gabriel Niebler der-gabe

----------------

* API changes

* Add new sort argument to simplify() to optionally not sort when simplifying
expressions (e.g. not applying "commutativity"). Thank you to Steven Esser
majurg for this
* Add new argument to tokenizer to optionally accept extra characters in symbol
tokens. Thank you to carpie for this

----------------

* No API changes

* Bug fixes

* Fix De Morgan's laws effect on double negation propositions. Thank you to Douglas Cardoso for this
* Improve error checking when parsing

-----------------

* No API changes

* Bug fixes

* Documentation updates and add testing for Python 3.6. Thank you to Alexander Lisianoi alisianoi
* Improve testng and expression equivalence checks
* Improve subs() method to an expression

------------------

* No API changes

* Bug fixes and improvements

* Fix various documentation typos and improve tests . Thank you to Alexander Lisianoi alisianoi
* Fix handling for literals vs. symbols in negations Thank you to YaronK