Bump six from 1.14.0 to 1.15.0 (#243)

requirements.txt

@@ -15,6 +15,6 @@ requests==2.24.0
 sciencebeam-alignment==0.0.5
 sciencebeam-utils==0.1.0
 sklearn-crfsuite==0.3.6
-six==1.14.0
+six==1.15.0
 tensorflow-transform==0.21.0
 tqdm==4.48.2