[gae.py] Recognize versions as "1.20-pre3" as valid.
[gae.py] Recognize versions as "1.20-pre3" as valid.

R=50b2a565e2e78c292794832469a30ce5abc9959c@google.com
BUG=1385526

Change-Id: Ic22c8cc6c9dde6583ddefea83a9626ebe73f4fe2
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-py/+/4035068
Reviewed-by: Yiwei Zhang <50b2a565e2e78c292794832469a30ce5abc9959c@google.com>

luci/luci-py,luci/luci-py,luci/luci-py,luci/luci-py