Bump version number to 1.21a16

Downgrade version number to 1.21a15, it was never released

Fix typo in image name for lint job

Update Django version info in the README

Bump version number to 1.21a12

Bump version number to 1.21a11

Bump version number to 1.21a10

Bump version number to 1.21a9

Bump version to 1.21a8

Bump version to 1.21a8