Comparison to string is broken for nullable int
#### Code Sample, a copy-pastable example if possible

```python
import pandas as pd
df = pd.DataFrame({'x': [1, None]})
df.x.astype(pd.Int64Dtype()) == 'a'
```
#### Problem description

The above raises TypeError on line 627 in integer.py:

```python
result[mask] = op_name == "ne"
```
Here, the result is bool. Here is the exception:

Traceback (most recent call last):
  File "<input>", line 1, in <module>
  File "C:\Python37\lib\site-packages\pandas\core\ops.py", line 1731, in wrapper
    return dispatch_to_extension_op(op, self, other)
  File "C:\Python37\lib\site-packages\pandas\core\ops.py", line 1220, in dispatch_to_extension_op
    res_values = op(new_left, new_right)
  File "C:\Python37\lib\site-packages\pandas\core\arrays\integer.py", line 564, in cmp_method
    result[mask] = True if op_name == 'ne' else False
TypeError: 'bool' object does not support item assignment

The above works  as expected for float64 and other numeric types.

#### Expected Output

0    False
1    False
Name: x, dtype: bool

#### Output of ``pd.show_versions()``

<details>

INSTALLED VERSIONS
------------------
commit           : None
python           : 3.7.0.final.0
python-bits      : 64
OS               : Windows
OS-release       : 10
machine          : AMD64
processor        : Intel64 Family 6 Model 158 Stepping 9, GenuineIntel
byteorder        : little
LC_ALL           : None
LANG             : None
LOCALE           : None.None
pandas           : 0.25.1
numpy            : 1.16.4
pytz             : 2019.1
dateutil         : 2.8.0
pip              : 19.2.3
setuptools       : 39.0.1
Cython           : None
pytest           : None
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : 4.3.4
html5lib         : None
pymysql          : None
psycopg2         : None
jinja2           : None
IPython          : None
pandas_datareader: None
bs4              : None
bottleneck       : None
fastparquet      : None
gcsfs            : None
lxml.etree       : 4.3.4
matplotlib       : 3.1.1
numexpr          : None
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : None
pytables         : None
s3fs             : None
scipy            : 1.3.0
sqlalchemy       : 1.3.5
tables           : None
xarray           : None
xlrd             : None
xlwt             : None
xlsxwriter       : None

</details>
