New license request: Add pretix AGPL exception

[pombredanne]

License Name
The full name of the license, preferably as provided by the original authors.

TBD

License Homepage URL
The primary and/or most reliable URL that contains the text of this license and/or information about it. This is ideally a web page dedicated to the license, but it may also be a source file that contains the license text.

https://github.com/pretix/pretix/blob/master/LICENSE#L1

Reference URLs
Optionally provide URLs that contain references to the license.

License Notes
Optionally describe any aspects of the license that deserve special attention.

Quite similar to #4587 but with different project names.... likely best to treat as two licenses. Here is an additionref for you! ;)

[DennisClark]

New license exception LicenseRef-scancode-pretix-exception-3.0 created in DejaCode enterprise and public. Copyleft.

@AyanSinhaMahapatra please synchronize when you have time, thanks.