df[col].astype('interval') no longer works for interval string
We used the below code to roundtrip pd.Interval values in 0.25.3, but it throws TypeError in 1.0.5:
```
df = pd.DataFrame(data={'i': [pd.Interval(0, 2, closed='left')]})
df['i'] = df['i'].astype(str) 
df['i'].astype('interval')
# throws: TypeError: type <class 'str'> with value [0, 2) is not an interval
```
We didn't use to_json/read_json because it is also not parsing pd.Interval back properly:
```
df = pd.DataFrame(data={'i': [pd.Interval(0, 2, closed='left')]})
value_json = df.to_json(orient='values')
pd.read_json(value_json, orient='values').values
# returns: array([[{'closed': 'left', 'closed_right': False, 'left': 0, 'mid': 1.0, 'open_right': True, 'right': 2}]],
      dtype=object)
```