Add function for xor gate (#7588)

* Add function for xor gate

* [pre-commit.ci] auto fixes from pre-commit.com hooks

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* Add test case for xor functions

* Update boolean_algebra/xor_gate.py

Co-authored-by: Christian Clauss <cclauss@me.com>

* Update boolean_algebra/xor_gate.py

Co-authored-by: Christian Clauss <cclauss@me.com>

* Split long comment line into two lines

* 88 characters per line

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Christian Clauss <cclauss@me.com>