[BugFix] Treasury Rates Pandas Warnings

[deeleeramone]

1. Why? (1-3 sentences or a bullet point list):

   • Warning being raised from a Pandas process.

2. What? (1-3 sentences or a bullet point list):

   • Made a copy of the DataFrame before the operation.

No more warning:

In [2]: obb.fixedincome.government.treasury_rates()
Out[2]: 
OBBject

id: 0663a69e-5065-7226-8000-dd5978363882
results: [{'date': datetime.date(2023, 5, 8), 'week_4': None, 'month_1': 0.05509999...
provider: federal_reserve
warnings: None
chart: None
extra: {'metadata': {'arguments': {'provider_choices': {'provider': 'federal_reserv...