Exclude "tests" package from dist

[mbargull]

This removes the included "tests" package from the built distribution.
Including it will cause file collisions (/overwrites) for site-packages/tests/__init__.py on installation.
Excluding it, however, means that the sdist also does not carry this file anymore, i.e., the setup_requires=["pytest-runner"] will not pick it up anymore -- which isn't much of a regression since the tests/*.sh files aren't included in the sdist anyway. (I.e., setup_requires=["pytest-runner"] is a quasi no-op apart from when run from the repo itself (or archive of it), so, I'd suggest to remove it from setup.py and invoke pytest directly.)

[johanneskoester]

Sounds reasonable, good catch. May I ask you to create a corresponding PR for removing the dependency (or just do it here).