Make python only `bzl_library` target publicly visible (#611)

Co-authored-by: Alex Eagle <eagle@post.harvard.edu>

python/BUILD

diff --git a/python/BUILD b/python/BUILD
index 7dbbec3..74c2c1c 100644
--- a/python/BUILD
+++ b/python/BUILD
@@ -49,7 +49,7 @@
         "//python/pip_install:bzl",
         "//python/private:bzl",
     ],
-    visibility = ["//:__pkg__"],
+    visibility = ["//visibility:public"],
 )
 
 # ========= Core rules =========