Add a MANIFEST.in to get a complete sdist

In particular, we now include test data in sdist, so that it's possible
to run tests from the tarball that'd be uploaded on PyPI. This is useful
for distribution packaging where tests can be run during package build.

Also include LICENSE and README files along the way.

Author: dlax

MANIFEST.in

@@ -0,0 +1,7 @@
+include README.md
+include LICENSE
+recursive-include test/data *.eml *.txt
+recursive-include test *.py
+
+prune .editorconfig
+prune .build.yml