bp2build support for python_test(_host)

There was a request for using b with python tests. bp2build python
tests exactly the same way as python binaries so that they can be
used with `b`.

Bug: None
Test: go test
Change-Id: Id68a6a73572745a4885b3e5bb1b8452e36baa982

python/python.go

@@ -239,6 +239,7 @@ var (
 	protoExt                 = ".proto"
 	pyVersion2               = "PY2"
 	pyVersion3               = "PY3"
+	pyVersion2And3           = "PY2ANDPY3"
 	internalPath             = "internal"
 )