* Upgrade Django to 3.0 and other dependencies to recent versions
* Fix otp version contsraint
* Remove six dependency
* Resolve some warnings
* Fix failing tests
* Update django-countries
* Resolve all RemovedInDjango31Warnings in test suite
* Run isort
* Fix import
* Update PostgreSQL version on travis
* Add testmode-support-flag to SalesChannels
* Make saleschannels/testmode-warnings even more dangerous!
* Add warning for payment-methods that do support testmode but are being used in a non-testmode order caused by a saleschannel in a testmode-shop.
* Remove redundant testmode_supported-flag for WebshopSalesChannel
* Raise error on API when sales_channel does not support testmode
* Tests
* Fix style issue after merge
- [x] Data model
- [x] Enforce constraint
- [x] Filter order list
- [x] Set channel on created order
- [x] Products API
- [x] Order API
- [x] Tests
- [x] Filter reports
- [x] Resellers
- [ ] deploy plugins
- [ ] posbackend
- [ ] resellers
- [ ] reports
- [x] Ticketlayouts
- [x] Support in pretixPOS