REGR: Change to Series[dt64/td64].astype("string")` behavior on master
xref https://github.com/pandas-dev/pandas/pull/38562#issuecomment-836438843 cc @jbrockmendel 

old behaviour

```
>>> pd.__version__
'1.2.4'
>>> 
>>> tdi = pd.timedelta_range("1 Day", periods=3)
>>> ser = pd.Series(tdi)
>>> ser
0   1 days
1   2 days
2   3 days
dtype: timedelta64[ns]
>>> ser.astype("string")
0    1 days
1    2 days
2    3 days
dtype: string
>>> 
>>> dti = pd.date_range("2021", periods=3)
>>> ser = pd.Series(dti)
>>> ser
0   2021-01-01
1   2021-01-02
2   2021-01-03
dtype: datetime64[ns]
>>> ser.astype("string")
0    2021-01-01
1    2021-01-02
2    2021-01-03
dtype: string
```

new behaviour

```
>>> pd.__version__
'1.3.0.dev0+1567.g67c9385787'
>>> 
>>> tdi = pd.timedelta_range("1 Day", periods=3)
>>> ser = pd.Series(tdi)
>>> ser
0   1 days
1   2 days
2   3 days
dtype: timedelta64[ns]
>>> ser.astype("string")
0     86400000000000 nanoseconds
1    172800000000000 nanoseconds
2    259200000000000 nanoseconds
dtype: string
>>> 
>>> dti = pd.date_range("2021", periods=3)
>>> ser = pd.Series(dti)
>>> ser
0   2021-01-01
1   2021-01-02
2   2021-01-03
dtype: datetime64[ns]
>>> ser.astype("string")
0    2021-01-01T00:00:00.000000000
1    2021-01-02T00:00:00.000000000
2    2021-01-03T00:00:00.000000000
dtype: string
>>> 
```