Update python

Changes:

- https://github.com/docker-library/python/commit/925dabd: Merge pull request https://github.com/docker-library/python/pull/439 from infosiftr/3.9-rc
- https://github.com/docker-library/python/commit/a7db8db: Add 3.9-rc builds

library/python

@@ -4,6 +4,31 @@ Maintainers: Tianon Gravi <[email protected]> (@tianon),
              Joseph Ferguson <[email protected]> (@yosifkit)
 GitRepo: https://github.com/docker-library/python.git
 
+Tags: 3.9.0a1-buster, 3.9-rc-buster, rc-buster
+SharedTags: 3.9.0a1, 3.9-rc, rc
+Architectures: amd64, arm32v5, arm32v7, arm64v8, i386, ppc64le, s390x
+GitCommit: a7db8db3c0eb92c86fbe74a91cb0eab4f7699a21
+Directory: 3.9-rc/buster
+
+Tags: 3.9.0a1-alpine3.10, 3.9-rc-alpine3.10, rc-alpine3.10, 3.9.0a1-alpine, 3.9-rc-alpine, rc-alpine
+Architectures: amd64, arm32v6, arm32v7, arm64v8, i386, ppc64le, s390x
+GitCommit: a7db8db3c0eb92c86fbe74a91cb0eab4f7699a21
+Directory: 3.9-rc/alpine3.10
+
+Tags: 3.9.0a1-windowsservercore-ltsc2016, 3.9-rc-windowsservercore-ltsc2016, rc-windowsservercore-ltsc2016
+SharedTags: 3.9.0a1-windowsservercore, 3.9-rc-windowsservercore, rc-windowsservercore, 3.9.0a1, 3.9-rc, rc
+Architectures: windows-amd64
+GitCommit: a7db8db3c0eb92c86fbe74a91cb0eab4f7699a21
+Directory: 3.9-rc/windows/windowsservercore-ltsc2016
+Constraints: windowsservercore-ltsc2016
+
+Tags: 3.9.0a1-windowsservercore-1809, 3.9-rc-windowsservercore-1809, rc-windowsservercore-1809
+SharedTags: 3.9.0a1-windowsservercore, 3.9-rc-windowsservercore, rc-windowsservercore, 3.9.0a1, 3.9-rc, rc
+Architectures: windows-amd64
+GitCommit: a7db8db3c0eb92c86fbe74a91cb0eab4f7699a21
+Directory: 3.9-rc/windows/windowsservercore-1809
+Constraints: windowsservercore-1809
+
 Tags: 3.8.0-buster, 3.8-buster, 3-buster, buster
 SharedTags: 3.8.0, 3.8, 3, latest
 Architectures: amd64, arm32v5, arm32v7, arm64v8, i386, ppc64le, s390x