dev-python/s3transfer: rectify metadata overwrite

xiamao pointed out that I accidentally threw this work overtop of
existing work.  He suggested I at least merge the metadata with that in
6d3d8a7 and this commit does that.

If there are any other problems due to this error, I'm sorry and feel
free to perform any necessary cleanups.

Package-Manager: portage-2.3.0

dev-python/s3transfer/metadata.xml

@@ -8,6 +8,10 @@
 		<email>[email protected]</email>
 		<name>Alex Brandt</name>
 	</maintainer>
+	<maintainer type="person">
+	  <email>[email protected]</email>
+	  <name>Tobias Klausmann</name>
+	</maintainer>
 	<longdescription lang="en">
 	</longdescription>
 </pkgmetadata>