Edit:
If you must use a for-loop, you can do this:
>>> tt = [2015, 2016, 2017, 2018, 2019, 2020, 2021, 2022, 2023, 2024]
>>> lst = []
>>> for t in tt:
... lst.append("Cal {}".format(t))
...
>>> lst
['Cal 2015', 'Cal 2016', 'Cal 2017', 'Cal 2018', 'Cal 2019', 'Cal 2020', 'Cal 2021', 'Cal 2022', 'Cal 2023', 'Cal 2024']
>>>
Use a list comprehension and str.format
:
>>> tt = [2015, 2016, 2017, 2018, 2019, 2020, 2021, 2022, 2023, 2024]
>>> ["Cal {}".format(t) for t in tt]
['Cal 2015', 'Cal 2016', 'Cal 2017', 'Cal 2018', 'Cal 2019', 'Cal 2020', 'Cal 2021', 'Cal 2022', 'Cal 2023', 'Cal 2024']
>>>