Make matplotlib dependency optional

[jakobkogler]

Currently if you install this library, you automatically install all it's dependencies like matplotlib and pyparsing. Those however are not needed for the core functionality (running architecture tests).

I would propose of making those diagram related libraries optional. It's possible to do that with extras in Poetry: https://python-poetry.org/docs/pyproject/#extras

• pip install pytestarch should install just the core functionality (calling .visualize() should raise some missing-libraries exception).
• pip install 'pytestarch[diagram]' should additionally install matplotlib.

[zyskarch]

I've released a new version (2.0.0), which makes this possible. Thanks for the suggestion!