Include test data in sdist (#98)

Explicitly include test data files (`*.txt`) in sdist archives.
Otherwise, only test Python files are included and the test suite
fails because of missing data files.

Author: mgorny

MANIFEST.in

@@ -4,3 +4,4 @@ include requirements-*.in requirements-*.txt
 recursive-include bin *.py
 recursive-include code_templates *.j2
 recursive-include docs *.py *.rst requirements.txt
+recursive-include tests *.txt