BUG: unpickle of Timestamp should not generate deprecation warning
Currently we give a deprecation warning when unpickling:

```
In [10]: idx = pd.date_range("2012", periods=3, freq="D")

In [11]: res = pickle.dumps(idx[0])

In [12]: pickle.loads(res)
/home/joris/miniconda3/envs/dev/bin/ipython:1: FutureWarning: The 'freq' argument in Timestamp is deprecated and will be removed in a future version.
  #!/home/joris/miniconda3/envs/dev/bin/python
Out[12]: Timestamp('2012-01-01 00:00:00', freq='D')
```

We should ensure that unpickling a Timestamp does not generate a warning, only if the user afterwards accesses the freq as normal it should warn.