tests: revert testing against prerelease deps on Python 3.9 (#527)

tseaver · web-flow · commit 33f00ab8245a · 2021-08-18T14:26:24.000Z
Consensus from today's meeting is that testing against prereleases needs to happen outside the normal presubmit path. Reverts only part of PR #479.
diff --git a/testing/constraints-3.9.txt b/testing/constraints-3.9.txt
@@ -1,2 +0,0 @@
-# Allow prerelease requirements
---pre

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