Mixed object array with astype to Extension Array throws TypeError
This works

```python
>>> pd.Series([1, "1"]).astype(int)
0    1
1    1
dtype: int64
```

This doesn't

```python
>>> pd.Series([1, "1"]).astype("Int64")
Traceback (most recent call last):
  File "/Users/willayd/clones/pandas/pandas/core/arrays/integer.py", line 165, in safe_cast
    return values.astype(dtype, casting="safe", copy=copy)
TypeError: Cannot cast array from dtype('O') to dtype('int64') according to the rule 'safe'

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/willayd/clones/pandas/pandas/core/generic.py", line 5635, in astype
    new_data = self._mgr.astype(dtype=dtype, copy=copy, errors=errors,)
  File "/Users/willayd/clones/pandas/pandas/core/internals/managers.py", line 573, in astype
    return self.apply("astype", dtype=dtype, copy=copy, errors=errors)
  File "/Users/willayd/clones/pandas/pandas/core/internals/managers.py", line 402, in apply
    applied = getattr(b, f)(**kwargs)
  File "/Users/willayd/clones/pandas/pandas/core/internals/blocks.py", line 581, in astype
    values = astype_nansafe(vals1d, dtype, copy=True)
  File "/Users/willayd/clones/pandas/pandas/core/dtypes/cast.py", line 912, in astype_nansafe
    return dtype.construct_array_type()._from_sequence(arr, dtype=dtype, copy=copy)
  File "/Users/willayd/clones/pandas/pandas/core/arrays/integer.py", line 362, in _from_sequence
    return integer_array(scalars, dtype=dtype, copy=copy)
  File "/Users/willayd/clones/pandas/pandas/core/arrays/integer.py", line 153, in integer_array
    values, mask = coerce_to_array(values, dtype=dtype, copy=copy)
  File "/Users/willayd/clones/pandas/pandas/core/arrays/integer.py", line 270, in coerce_to_array
    values = safe_cast(values, dtype, copy=False)
  File "/Users/willayd/clones/pandas/pandas/core/arrays/integer.py", line 172, in safe_cast
    raise TypeError(
TypeError: cannot safely cast non-equivalent object to int64
```
