Increment juju devel to 1.21-alpha1.
Merge pull request #242 from sinzui/inc-1.21-alpha1

Increment juju devel to 1.21-alpha1.

The CI publish-revision test fails early because it knows it cannot test a devel version that is lesser than the stable version. This branch updates the devel version and windows installer to 1.21-alpha1 (to match the current milestone).