Avoid creating unnecessary objects by using generator expressions

[richcooper95]

Describe the bug
In some places, objects are created unnecessarily - e.g. here:

if set(payment.keys()) != set(["amount", "currency", "type"]):

To Reproduce
n/a

Expected behavior
There are slight clarity, performance and memory gains by avoiding unnecessary creation via use of generator expressions - e.g. the above could be changed to:

if not all(k in ["amount", "currency", "type"] for k in payments.keys()):

which only creates a single list, rather than two sets and a list.

System (please complete the following information):
n/a

Additional context
Over such small sizes of list/set it won't make a tangible performance impact, so this is pretty low-priority - but I think it also makes the code a bit clearer.