Add a new request type with its own StarLark function
Hook it up via GetPythonBinary
Add to MockBazelContext a LabelToPythonBinary
Add a test for the new request type
Test: request_type_test.go:TestGetPythonBinaryParseResults
Change-Id: I05f6506adfbbdac8b3f40475509ed02ab8e844e5