Add py.typed to MANIFEST.in (#207)

This ensures that the py.typed marker is included in source
distributions and wheels.

Co-authored-by: Darryl Ring <10316@redlen.com>

MANIFEST.in

@@ -1,5 +1,6 @@
 include LICENSE
 include LICENSE-3RD-PARTY
 include systemrdl/preprocessor/ppp_runner.pl
+include systemrdl/py.typed
 recursive-include systemrdl/parser/ext *
-recursive-exclude test *
+recursive-exclude test *