BUG: isin does some inconsistent type casting?
- [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. (exists on the version I checked out 2 weeks ago)

I observed the following behavior: 

When I define my `DataFrame` with a numeric column like

```
import pandas as pd
df = pd.DataFrame({"a": [1, 2]})
```

and filter with `isin()` for `"1"` like

```
df["a"].isin(["1"])
```

I get

```
0     True
1    False
Name: a, dtype: bool
```

which results probably from implicit type casting?

But when my column has datatype object like

```
df = pd.DataFrame({"a": [1, "2"]})
```

and filter again with `isin()`

```
df["a"].isin(["1"])
```

I get the following:

```
0    False
1    False
Name: a, dtype: bool
```

That was the output I expected.
This seems a bit inconsistent. Is this the expected behavior?  If not, I would look into this, if I have a bit of time in the coming weeks. This may be loosely related to #19356 . 


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

<details>

python           : 3.7.7.final.0
python-bits      : 64
OS               : Linux
OS-release       : 4.15.0-99-generic
machine          : x86_64
processor        : x86_64
byteorder        : little
LC_ALL           : None
LANG             : en_US.UTF-8
LOCALE           : en_US.UTF-8

pandas           : 1.0.1
numpy            : 1.18.1
pytz             : 2019.3
dateutil         : 2.8.1
pip              : 20.0.2
setuptools       : 46.1.3.post20200330
Cython           : 0.29.15
pytest           : 5.4.1
hypothesis       : 5.8.3
sphinx           : 2.4.4
blosc            : None
feather          : None
xlsxwriter       : 1.2.8
lxml.etree       : 4.5.0
html5lib         : 1.0.1
pymysql          : None
psycopg2         : None
jinja2           : 2.11.1
IPython          : 7.13.0
pandas_datareader: None
bs4              : 4.8.2
bottleneck       : 1.3.2
fastparquet      : None
gcsfs            : None
lxml.etree       : 4.5.0
matplotlib       : 3.1.3
numexpr          : 2.7.1
odfpy            : None
openpyxl         : 3.0.3
pandas_gbq       : None
pyarrow          : None
pytables         : None
pytest           : 5.4.1
pyxlsb           : None
s3fs             : None
scipy            : 1.4.1
sqlalchemy       : 1.3.13
tables           : 3.6.1
tabulate         : None
xarray           : None
xlrd             : 1.2.0
xlwt             : 1.3.0
xlsxwriter       : 1.2.8
numba            : 0.49.0


</details>
