tests: allow prerelease deps on Python 3.9 (#402)

tseaver · web-flow · commit d8532682dbdd · 2021-08-17T12:30:11.000-04:00
Closes #401.
diff --git a/testing/constraints-3.9.txt b/testing/constraints-3.9.txt
@@ -0,0 +1,2 @@
+# Allow prerelease requirements
+--pre

Original file line number	Diff line number	Diff line change
`@@ -0,0 +1,2 @@`
	`1`	`+# Allow prerelease requirements`
	`2`	`+--pre`