REGR: ExtensionArray aggregation on non-numeric types fails
- [X] I have checked that this issue has not already been reported.

- [X] I have confirmed this bug exists on the latest version of pandas.

- [X] (optional) I have confirmed this bug exists on the master branch of pandas.

---

#### Code Sample, a copy-pastable example

```python
df = pd.DataFrame({'key': ['a', 'a', 'b', 'b'], 'val': ['a', 'b', 'c', 'd']}, dtype="string")
df.groupby("key").agg({'val': 'first'})
```

Expected:
```
    val
key    
a     a
b     c
```

Actual fails with error:
```
lib/python3.8/site-packages/pandas/core/groupby/ops.py in _ea_wrap_cython_operation(self, kind, values, how, axis, min_count, **kwargs)
    538             return result
    539 
--> 540         raise NotImplementedError(values.dtype)
    541

NotImplementedError: string
```

#### Problem description

Calling groupby on an ExtensionArray and aggregation on a string column fails with the error: `NotImplementedError: string`. This is because an optimized cython aggregation is attempted and when fails, the error string is improperly checked and does not fallback to a non-cython aggregation.

This was found with the extension arrays from https://github.com/CODAIT/text-extensions-for-pandas/blob/master/text_extensions_for_pandas/io/bert.py#L217  and full error log can be seen here https://github.com/CODAIT/text-extensions-for-pandas/pull/157/checks?check_run_id=1646927381#step:5:1438

#### Expected Output

Pandas should handle the NotImplementedError and fallback to a non-cython aggregation implementation.

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

<details>

INSTALLED VERSIONS
------------------
commit           : 3e89b4c4b1580aa890023fc550774e63d499da25
python           : 3.8.6.final.0
python-bits      : 64
OS               : Linux
OS-release       : 5.4.0-58-generic
Version          : #64-Ubuntu SMP Wed Dec 9 08:16:25 UTC 2020
machine          : x86_64
processor        : x86_64
byteorder        : little
LC_ALL           : None
LANG             : en_US.UTF-8
LOCALE           : en_US.UTF-8

pandas           : 1.2.0
numpy            : 1.19.1
pytz             : 2020.1
dateutil         : 2.8.1
pip              : 20.2.4
setuptools       : 49.6.0.post20201009
Cython           : None
pytest           : 6.1.1
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : None
html5lib         : None
pymysql          : None
psycopg2         : None
jinja2           : 2.11.2
IPython          : 7.18.1
pandas_datareader: None
bs4              : None
bottleneck       : None
fsspec           : None
fastparquet      : None
gcsfs            : None
matplotlib       : 3.3.2
numexpr          : None
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : 2.0.0
pyxlsb           : None
s3fs             : None
scipy            : 1.5.2
sqlalchemy       : None
tables           : None
tabulate         : None
xarray           : None
xlrd             : None
xlwt             : None
numba            : None

</details>
