Merge pull request #6021 from grpc/release-0_13

0.13 a tiny upmerge 
diff --git a/PYTHON-MANIFEST.in b/PYTHON-MANIFEST.in
index 25aba87..534f4c1 100644
--- a/PYTHON-MANIFEST.in
+++ b/PYTHON-MANIFEST.in
@@ -2,8 +2,10 @@
 recursive-exclude src/python/grpcio/grpc/_cython *.so *.pyd
 graft src/python/grpcio/tests
 graft src/core
+graft src/boringssl
 graft include/grpc
 graft third_party/boringssl
+graft third_party/nanopb
 graft third_party/zlib
 include src/python/grpcio/commands.py
 include src/python/grpcio/grpc_version.py