MAINT: add cython-lint to test suite

[fkuehlein]

As discussed in #195, it would be helpful to extend the style and lint checking to the Cython modules. This comment kindly provides a TODO list already:

You could add cython-lint to our test suite by:

• adding it to the testing dependencies in setup.cfg,
• adding a new tox test environment in setup.cfg which calls cython-lint,
• and configuring cython-lint itself in pyproject.toml.

Note that this will inevitably lead to lots of flags and errors to be resolved.

See the cython-lint repo for reference.