Release 1.3.0a1 - alpha quality, for testing only

borgbackup 1.3.0a1 alpha release - do not use this on production backup repositories.

Long changelog:
https://borgbackup.readthedocs.io/en/1.3.0a1.post3/changes.html#version-1-3-0a1-2022-04-15

If you use pip to install this, use: pip install "borgbackup==1.3.0a1"

Docs: https://borgbackup.readthedocs.io/en/1.3.0a1.post3/

Feedback:

• discussion: #6597