temperature.fuentes errors when given tz-aware inputs on pandas>=1.0.0

[kandersolar]

Describe the bug
When the weather timeseries inputs to temperature.fuentes have tz-aware index, an internal call to np.diff(index) returns an array of Timedelta objects instead of an array of nanosecond ints, throwing an error immediately after. The error only happens when using pandas>=1.0.0; using 0.25.3 runs successfully, but emits the warning:

  /home/kevin/anaconda3/envs/pvlib-dev/lib/python3.7/site-packages/numpy/lib/function_base.py:1243: FutureWarning: Converting timezone-aware DatetimeArray to timezone-naive ndarray with 'datetime64[ns]' dtype. In the future, this will return an ndarray with 'object' dtype where each element is a 'pandas.Timestamp' with the correct 'tz'.
  	To accept the future behavior, pass 'dtype=object'.
  	To keep the old behavior, pass 'dtype="datetime64[ns]"'.
    a = asanyarray(a)

To Reproduce

In [1]: import pvlib
   ...: import pandas as pd
   ...: 
   ...: index_naive = pd.date_range('2019-01-01', freq='h', periods=3)
   ...: 
   ...: kwargs = {
   ...:     'poa_global': pd.Series(1000, index_naive),
   ...:     'temp_air': pd.Series(20, index_naive),
   ...:     'wind_speed': pd.Series(1, index_naive),
   ...:     'noct_installed': 45
   ...: }
   ...: 

In [2]: print(pvlib.temperature.fuentes(**kwargs))
2019-01-01 00:00:00    47.85
2019-01-01 01:00:00    50.85
2019-01-01 02:00:00    50.85
Freq: H, Name: tmod, dtype: float64

In [3]: kwargs['poa_global'].index = index_naive.tz_localize('UTC')
   ...: print(pvlib.temperature.fuentes(**kwargs))
   ...: 
Traceback (most recent call last):

  File "<ipython-input-3-ff99badadc91>", line 2, in <module>
    print(pvlib.temperature.fuentes(**kwargs))

  File "/home/kevin/anaconda3/lib/python3.7/site-packages/pvlib/temperature.py", line 602, in fuentes
    timedelta_hours = np.diff(poa_global.index).astype(float) / 1e9 / 60 / 60

TypeError: float() argument must be a string or a number, not 'Timedelta'

Expected behavior
temperature.fuentes should work with both tz-naive and tz-aware inputs.

Versions:

• pvlib.__version__: 0.8.0
• pandas.__version__: 1.0.0+
• python: 3.7.4 (default, Aug 13 2019, 20:35:49) \n[GCC 7.3.0]