gnu: python-rpy2: Update to 3.0.4-1.19868a8.

* gnu/packages/statistics.scm (python-rpy2): Update to 3.0.4-1.19868a8.
[source]: Fetch from repository.
[propagated-inputs]: Add python-cffi, python-numpy, python-pandas,
python-ipython, and python-tzlocal.
[native-inputs]: Add python-pytest.
[arguments]: Set TZ variable and use pytest to run the tests.