Add encrypt dict using property decorator

[benjamin-awd]

Description of the changes

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Additional comments

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[benjamin-awd]

Otherwise if calling self.encrypt_dict twice is an issue:

@property
def security_handler(self):
    if (encrypt_dict := self.encrypt_dict) and not self._security_handler:
        self._security_handler = SecurityHandler.build(encrypt_dict)
    return self._security_handler

[MatthiasValvekens]

Did you see #343? :)

EDIT: oops, didn't spot that this was a diff against that branch. Was replying on mobile at the time.

[benjamin-awd]

Did you see #343? :)

Ah yes I did -- just raising this as a diff, feel free to disregard this PR completely