Bump attrs from 19.2.0 to 19.3.0 (#284)

dependabot-preview[bot] · web-flow · commit 386ea1eba421 · 2019-10-15T22:26:04.000Z
diff --git a/requirements-dev.txt b/requirements-dev.txt
@@ -1,4 +1,4 @@
-attrs==19.2.0
+attrs==19.3.0
 flake8==3.7.8
 coverage==4.5.4
 sphinx==2.2.0
