BUG: Series.groupby fails with InvalidIndexError on time series with a tuple-named grouper.
- [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.

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

---

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

```python
import pandas as pd

s = pd.Series(index=[pd.Timestamp(2021,7,26)], name=('A', 1))
s.groupby(s==s)  # raises InvalidIndexError
```

#### Problem description

There are several things necessary to trigger this:
- The index has to consist of `pd.Timestamp`s. If you change the example to `s = pd.Series(index=[0,1], name=('A', 1))` everything is fine.
- The grouper, i.e. `groupby`'s `by` argument , has to be a `Series` named with a tuple (like you get if you select a `MultiIndex`-column from a `DataFrame`). If you change the example to `s.groupby((s==s).rename('foo'))` everything is fine. Same goes if you use a pure list or a function.
- The object to group must be a `Series`. If you change the example to `pd.DataFrame(s).groupby(s==s)` everything is fine.
- You need `pandas>=1.1.3`. If you downgrade to an older version - guess what - everything is fine.

Why this is a problem: The name of a grouper should not matter and it is just not to understand why a time series behaves differently than any other kind of `Series`.

#### Expected Output

Behavior should be as for non-time-series.

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

<details>

INSTALLED VERSIONS
------------------
commit           : db08276bc116c438d3fdee492026f8223584c477
python           : 3.8.3.final.0
python-bits      : 64
OS               : Windows
OS-release       : 10
Version          : 10.0.19041
machine          : AMD64
processor        : Intel64 Family 6 Model 158 Stepping 13, GenuineIntel
byteorder        : little
LC_ALL           : None
LANG             : None
LOCALE           : de_DE.cp1252
pandas           : 1.1.3
numpy            : 1.20.3
pytz             : 2021.1
dateutil         : 2.8.2
pip              : 21.1.3
setuptools       : 52.0.0.post20210125
Cython           : None
pytest           : 6.2.4
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : None
html5lib         : None
pymysql          : None
psycopg2         : None
jinja2           : None
IPython          : None
pandas_datareader: None
bs4              : None
bottleneck       : 1.3.2
fsspec           : None
fastparquet      : None
gcsfs            : None
matplotlib       : None
numexpr          : 2.7.3
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : None
pytables         : None
pyxlsb           : None
s3fs             : None
scipy            : None
sqlalchemy       : None
tables           : None
tabulate         : None
xarray           : None
xlrd             : None
xlwt             : None
numba            : None
</details>
