BUG: Can't compute group mean of complex values
### 

- [X] I have checked that this issue has not already been reported.

- [X] I have confirmed this bug exists on the [latest version](https://pandas.pydata.org/docs/whatsnew/index.html) of pandas.

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


### Reproducible Example

```python
import numpy as np
import pandas as pd

rng = np.random.default_rng(42)
data = pd.Series(rng.uniform(size=(10, 2)).dot([1, 1j]))
gr = data.groupby(data.index % 2)
gr.mean()  # TypeError: No matching signature found
```


### Issue Description

`gr.mean()` raises `TypeError: No matching signature found` when the underlying dtype is `complex128`. The other complex dtypes present the same behavior.

```python
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/goyo/.local/lib/python3.9/site-packages/pandas/core/groupby/groupby.py", line 1684, in mean
    result = self._cython_agg_general(
  File "/home/goyo/.local/lib/python3.9/site-packages/pandas/core/groupby/generic.py", line 352, in _cython_agg_general
    result = array_func(objvals)
  File "/home/goyo/.local/lib/python3.9/site-packages/pandas/core/groupby/generic.py", line 340, in array_func
    result = self.grouper._cython_operation(
  File "/home/goyo/.local/lib/python3.9/site-packages/pandas/core/groupby/ops.py", line 978, in _cython_operation
    return cy_op.cython_operation(
  File "/home/goyo/.local/lib/python3.9/site-packages/pandas/core/groupby/ops.py", line 639, in cython_operation
    return self._cython_op_ndim_compat(
  File "/home/goyo/.local/lib/python3.9/site-packages/pandas/core/groupby/ops.py", line 481, in _cython_op_ndim_compat
    res = self._call_cython_op(
  File "/home/goyo/.local/lib/python3.9/site-packages/pandas/core/groupby/ops.py", line 557, in _call_cython_op
    func(result, counts, values, comp_ids, min_count)
  File "pandas/_libs/groupby.pyx", line 668, in pandas._libs.groupby.__pyx_fused_cpdef
TypeError: No matching signature found
```

### Expected Behavior

Since `data.mean()` works I would expect that it also works in the `GroupBy` object. The expected result is:

```python
>>> gr.mean()
0    0.438939+0.550293j
1    0.652316+0.653820j
dtype: complex128
```

Just like `.sum()`:

```python
>>> gr.sum()  # this does work.
0    2.194697+2.751466j
1    3.261581+3.269100j
dtype: complex128
```




### Installed Versions

<details>

INSTALLED VERSIONS
------------------
commit           : 73c68257545b5f8530b7044f56647bd2db92e2ba
python           : 3.9.5.final.0
python-bits      : 64
OS               : Linux
OS-release       : 5.11.0-34-generic
Version          : #36-Ubuntu SMP Thu Aug 26 19:22:09 UTC 2021
machine          : x86_64
processor        : x86_64
byteorder        : little
LC_ALL           : None
LANG             : en_US.UTF-8
LOCALE           : en_US.UTF-8

pandas           : 1.3.3
numpy            : 1.21.2
pytz             : 2021.1
dateutil         : 2.8.2
pip              : 21.2.4
setuptools       : 58.0.4
Cython           : None
pytest           : None
hypothesis       : None
sphinx           : 3.5.3
blosc            : None
feather          : None
xlsxwriter       : 3.0.1
lxml.etree       : 4.6.3
html5lib         : 1.1
pymysql          : None
psycopg2         : None
jinja2           : 3.0.1
IPython          : 7.27.0
pandas_datareader: None
bs4              : 4.10.0
bottleneck       : 1.3.2
fsspec           : None
fastparquet      : None
gcsfs            : None
matplotlib       : 3.4.3
numexpr          : 2.7.3
odfpy            : None
openpyxl         : 3.0.8
pandas_gbq       : None
pyarrow          : None
pyxlsb           : None
s3fs             : None
scipy            : 1.7.1
sqlalchemy       : None
tables           : 3.6.1
tabulate         : None
xarray           : None
xlrd             : 2.0.1
xlwt             : 1.3.0
numba            : None


</details>
