[IMP] purchase: Convert yml files to python and xml

addons/purchase/__manifest__.py

addons/purchase/data/purchase_data.xml

addons/purchase/data/purchase_data.yml

addons/purchase/data/purchase_demo.xml

addons/purchase/data/purchase_order_demo.yml

addons/purchase/data/purchase_stock_demo.yml

addons/purchase/test/average_price.yml

addons/purchase/test/fifo_price.yml

addons/purchase/test/fifo_returns.yml

addons/purchase/test/process/cancel_order.yml

addons/purchase/test/process/run_scheduler.yml

addons/purchase/test/stock_valuation_account.xml

addons/purchase/test/ui/delete_order.yml

addons/purchase/test/ui/duplicate_order.yml

addons/purchase/test/ui/purchase_users.yml

addons/purchase/tests/__init__.py

addons/purchase/tests/common.py

addons/purchase/tests/test_average_price.py

addons/purchase/tests/test_fifo_price.py

addons/purchase/tests/test_fifo_returns.py