fix: include more files in sdist explicitly (#350)

Author: mgorny

MANIFEST.in

@@ -0,0 +1,6 @@
+# include all test files but avoid including .pyc
+recursive-include benchmark *.py *.txt
+recursive-include tests *.json *.html *.md *.py *.txt
+# include documentation sources but not built docs
+graft docs
+prune docs/_build