Modified requirements.txt and tests. #6346

1. tryton-sale-shop requirement modified. Needed, otherwise trytond
3.4.0 was being installed as dependency.
2. Made context a keyword argument in test transactions.
3. Fixed authorize.net issues.
Merge pull request #75 from PritishC/requirements-hotfix

Modified requirements.txt. #6346
Notes now cannot be added to a cancelled order. #6283

Added small test for checkout.