BUG: aggfunc use different default arguments in pivot_table
- [x] I have checked that this issue has not already been reported.

- [x] I have confirmed this bug exists on the latest version of pandas.

- [ ] (optional) I have confirmed this bug exists on the master branch of pandas.

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**Note**: Please read [this guide](https://matthewrocklin.com/blog/work/2018/02/28/minimal-bug-reports) detailing how to provide the necessary information for us to reproduce your bug.

#### Code Sample, a copy-pastable example

```python
import pandas as pd
import numpy as np
df = pd.DataFrame({'A': ['a', 'a'], 'X': [5, 9]})

# standard deviation of X:
np.std(df['X'])
# 2.0

# aggfunc returns the same standard deviation as np.std as expected with this trick
pd.pivot_table(df, index='A', values='X', aggfunc=lambda x: np.std(x))
#        X
#  A
#  a     2

# aggfunc returns the unbiased standard deviation unexpectedly
pd.pivot_table(df, index='A', values='X', aggfunc=np.std)

#               X
#  A
#  a     2.828427
```

#### Problem description
The default value of ddof for np.std is 0:
numpy.std(a, axis=None, dtype=None, out=None, ddof=0, keepdims=<no value>)

When np,std passed to aggfunc to calculate the standard deviation of X it returns the unbiased standard deviation, because it picks different ddof =1 and does not pick up the default ddof = 0.
On the other hand the expected behavior if we have a function f, when we pass it to aggfunc:
aggfunc=f and aggfunc=lambda x: f(x) must return exactly the same result. 


#### Expected Output

1) When  np.std is passed to aggfunc directly it picks up ddof=1 by default (this is unexpected as the default ddof for np.std is 0).
2) When np.std is passed to aggfunc by using lambda x: np.std(x), it picks up correctly the default ddof=0 from numpy as expected.

To summerize, the expected behavior is to use the function's default arguments when it is passed to aggregate values in pd.pivot_table.

#### Output of ``pd.show_versions()``

<details>

INSTALLED VERSIONS
------------------
commit           : 2a7d3326dee660824a8433ffd01065f8ac37f7d6
python           : 3.8.5.final.0
python-bits      : 32
OS               : Windows
OS-release       : 10
Version          : 10.0.19041
machine          : AMD64
processor        : Intel64 Family 6 Model 142 Stepping 12, GenuineIntel
byteorder        : little
LC_ALL           : None
LANG             : None
LOCALE           : English_United States.1252
pandas           : 1.1.2
numpy            : 1.19.2

</details>
