BUG: invalid type annotation for equal operator of Series
- [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.

---

**Note**: Please read [this guide](https://matthewrocklin.com/blog/work/2018/02/28/minimal-bug-reports) detailing how to provide the necessary information for us to reproduce your bug.

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

```python
import pandas as pd

df = pd.DataFrame(
	{'kind' : ['a', 'b', 'a'], 
	 'value' : [1, 2, 3]})

res: pd.Series = df['kind'] == 'a'
print(type(res))
```

#### Problem description

`mypy` returns the following error:

```
$ mypy test.py 
test.py:7: error: Incompatible types in assignment (expression has type "bool", variable has type
"Series")
    res: pd.Series = df['kind'] == 'a'
                     ^
Found 1 error in 1 file (checked 1 source file)
$ mypy --version
mypy 0.812
```

But actual type of the result at runtime is `Series`:

```
$ python test.py 
<class 'pandas.core.series.Series'>
```

#### Expected Output

No compile errors.

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

<details>

```
>>> pd.show_versions()

INSTALLED VERSIONS
------------------
commit           : f2c8480af2f25efdbd803218b9d87980f416563e
python           : 3.8.5.final.0
python-bits      : 64
OS               : Linux
OS-release       : 5.4.0-70-generic
Version          : #78-Ubuntu SMP Fri Mar 19 13:29:52 UTC 2021
machine          : x86_64
processor        : x86_64
byteorder        : little
LC_ALL           : None
LANG             : en_US.UTF-8
LOCALE           : en_US.UTF-8

pandas           : 1.2.3
numpy            : 1.20.2
pytz             : 2021.1
dateutil         : 2.8.1
pip              : 20.0.2
setuptools       : 44.0.0
Cython           : None
pytest           : None
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       : None
fsspec           : None
fastparquet      : None
gcsfs            : None
matplotlib       : None
numexpr          : None
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : None
pyxlsb           : None
s3fs             : None
scipy            : None
sqlalchemy       : None
tables           : None
tabulate         : None
xarray           : None
xlrd             : None
xlwt             : None
numba            : None
```

</details>
