Bump six requirement

ryanfaircloth · web-flow · commit a2ce00306aa7 · 2020-10-29T10:40:17.000+03:00
collections_abc was added to six in 1.15 and is in use by this package but was not covered prior. Centos 8 includes six.py 1.13 causing an error
diff --git a/requirements.txt b/requirements.txt
@@ -1,4 +1,4 @@
 requests>=2.23.0
-six>=1.13.0
+six>=1.15.0
 pytest
 delayed-assert
