Feature Request: context manager in which NaN equals NaN
I'm not really sure whether this is the correct place to request for this (maybe it should be numpy?), but here it is.

Often times when we deal with data we may want to compare two `Series` or `DataFrames`. In pandas/numpy standard and many other, if not all, programming standards, `NaN == NaN` yields ``False``. 

However, sometimes we really mean to make them equal. Say we are comparing two raw datasets. In this case it'd be nice if we have a context manager within which `NaN == NaN` yields True.
```python
>>> import numpy as np
>>> import pandas as pd
>>> pd.__version__
'0.25.3'

>>> s1 = pd.Series([1, 2, np.nan])
>>> s2 = pd.Series([1, 3, np.nan])

# Current behavior
>>> s1 == s2
0     True
1    False
2    False
dtype: bool

# Desired feature
>>> with pd.make_nans_equal():
>>>     print(s1 == s2)
0     True
1    False
2     True
dtype: bool
```

In addition, it'd be nice if we can configure the proposed `pd.make_nans_equal()` with parameters to set if we want `NaN == None` to be ``True`` too.
