Replace deprecated setup.py approach and nosetests.

[mateuszmandera]

Closes #31

• installing via setup.py is deprecated and https://packaging.python.org/en/latest/tutorials/packaging-projects/ recommends using a pyproject.toml file for the kinds of things we currently specify in setup.py
• nose is unmaintained and doesn't even work on newer python versions. nose2 is a replacement, but not similar enough. pytest is much more standard nowadays, so let's switch to using that.

Thus this commit does two things:

1. Get rid of setup.py and make a pyproject.toml file.
2. Replace nose with pytest.

Ideally these two changes would be split into separate commits, but they depend on each other:

• I don't know how to convert setup.py to pyproject.toml while keeping the nose-specific config.
• pytest says clearly (https://docs.pytest.org/en/latest/explanation/goodpractices.html#do-not-run-via-setuptools) not to integrate with setuptools.

eq_ and assert_raises were imported from nose and instead unittest equivalents should be used.

README is updated to represent the current situation better.

[timabbott]

Looks good, merged, thanks @mateuszmandera!