Release 0.13.0

## Packaging
- Drop the dependency on leapp-repository for Fedora and RHEL 8+ (oamg#717)
- Provide builds for RHEL 7+ and Fedora (oamg#717)
- Drop automatically generated Python dependences on RHEL 8+ and Fedora systems (oamg#717, oamg#716)
- Bump the provided leapp-framework capability to 2.0 (oamg#700)

## Framework
### Fixes
- models: Do not make references to private symbols (oamg#718)

### Enhancements
- Introduce the LEAPP_DEVEL_DATABASE_SYNC_OFF envar to enable speed up writes into the leapp database by disabling synchronisation - only for development / testing purposes (oamg#732)

## Leapp
### Fixes
- Fix print of the leapp help msg for Python 3.6+ (oamg#731)

### Enhancements
- The leapp commands are now defined/provided by leapp-repositories; leapp discovers the specified commands automatically (oamg#700)
- Add CLI support for `choices` and `default` for options of leapp commands (oamg#734)

## Modifications
- Makefile: Added the `fast_lint` target to apply linters just on files different from master (oamg#733)