Testing of sdist fails

[mtelka]

When I test sdist from PyPI it always fails:

ImportError while loading conftest '$(BUILDDIR)/test/conftest.py'.
test/conftest.py:11: in <module>
    os.path.join(TEST_DATA_DIR, p) for p in os.listdir(TEST_DATA_DIR)
E   FileNotFoundError: [Errno 2] No such file or directory: '$(BUILDDIR)/libbrotli/tests/testdata'

Apparently, the required directory is not in the sdist. Please add it there. Please also consider to add tox.ini to sdist.

Thank you.