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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def verify_claims(app_req, issuer=None):
 """ Verify JWT claims. All times must be UTC unix timestamps. These claims will be verified: - iat: issued at time. If JWT was issued more than an hour ago it is rejected. - exp: expiration time. All exceptions are derived from :class:`mozpay.exc.InvalidJWT`. For expirations a :class:`mozpay.exc.RequestExpired` exception will be raised. """

    if not issuer:
        issuer = _get_issuer(app_req=app_req)
    try:
        float(str(app_req.get('exp')))
        float(str(app_req.get('iat')))
    except ValueError:
        _re_raise_as(InvalidJWT,
                     'JWT had an invalid exp (%r) or iat (%r) '
                     % (app_req.get('exp'), app_req.get('iat')),
                     issuer=issuer)