Avoid using mutable defaults.

[imkaka]

Mutable {} is used in functions everywhere in the project, which can lead to leaks if these functions are not called properly.
refer - https://florimond.dev/blog/articles/2018/08/python-mutable-defaults-are-the-source-of-all-evil/

def capture(self, payment_id, amount, data={}, **kwargs):

def refund(self, payment_id, amount, data={}, **kwargs):  # pragma: no cover

[vku2018]

Just to add some concrete impact here: this actually bites any setup that shares a single Client across a threaded server (the usual pattern). capture()'s data={} default is created once and then mutated on every call (data['amount'] = amount), so two threads capturing different payments race on the same dict — one payment can go out with another's amount. Single-threaded it stays hidden because the key just gets overwritten each time.

I put up #331 with a fix (switch to data=None + build a fresh dict per call, which also stops mutating a caller-supplied dict) plus a small concurrency repro and regression tests. Happy to adjust the approach if you'd prefer something else.