1.9.15

• commit 7683286 (HEAD -> 1.9.x, origin/1.9.x)
  | Author: Iggy Eom [email protected]
  | Date: Wed Jun 17 17:03:10 2020 +0900
  |
  | Bump Django version to 1.9.15.
  |
• commit 1cb5bc2
  | Author: Iggy Eom [email protected]
  | Date: Wed Jun 17 16:36:07 2020 +0900
  |
  | Suppress nullable user.last_login.
  |* commit a20ce1a
  | Author: Iggy Eom [email protected]
  | Date: Wed May 20 16:31:47 2020 +0900
  |
  | Revert microsecond precision support.
  |
  | This commit partially reverts 22da5f8
  |
• commit 82036a5
  | Author: Oliver Chang [email protected]
  | Date: Sat Apr 4 16:37:18 2020 -0700
  |
  | Fixed Python 3.6 deprecation warning for empty model super() calls.
  |
  | See: django@dd99e69