Error when adding 'm' to a Series of strings
#### Code Sample

I have found a strange corner case when adding strings to a Series. This is an operation that most of the times works well:
```python
import pandas as pd

# Adding a string to a Series, works as expected
pd.Series(['hello','world'])+'!'
```
output:
```
0    hello!
1    world!
dtype: object
```
But in the specific case of adding the string 'm', it raises an exception:

```python
# Adding the string 'm' to a Series
pd.Series(['hello','world'])+'m'
```
output:
```
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-125-795dd84b51d1> in <module>
----> 1 pd.Series(['hello','world'])+'m'

~/PycharmProjects/nsa.notebooks-voice/virtual3/lib/python3.6/site-packages/pandas-0.25.1-py3.6-macosx-10.12-x86_64.egg/pandas/core/ops/__init__.py in wrapper(left, right)
   1014             if is_scalar(right):
   1015                 # broadcast and wrap in a TimedeltaIndex
-> 1016                 assert np.isnat(right)
   1017                 right = np.broadcast_to(right, left.shape)
   1018                 right = pd.TimedeltaIndex(right)

TypeError: ufunc 'isnat' is only defined for datetime and timedelta.

```
#### Problem description

It took me a while to figure this one out, but looking at the [source code ](https://github.com/pandas-dev/pandas/blob/171c71611886aab8549a8620c5b0071a129ad685/pandas/core/ops/__init__.py) of _arith_method_SERIES() in pandas/pandas/core/ops/__init__.py, it's apparently due to running is_timedelta64_dtype() as part of type checks for the scalar. String 'm' is incorrectly parsed as a timedelta64.

```python
pd.core.ops.is_timedelta64_dtype('m')
>True
```
This can be traced to a pandas_dtype() function:

```python
pd.core.dtypes.common.pandas_dtype('m')
>dtype('<m8')
```
Presumably there are other random strings with similar interactions with type checking functions that throw similar errors in this operation.

I get this error on build 25.1 . I haven't been able to test this on the latest master code, which introduces a lot of changes to /pandas/core/ops/__init__ , so I don't know if this is still a problem there.


#### Expected Output

Adding strings to a string Series shouldn't throw this exception.

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

<details>

INSTALLED VERSIONS
------------------
commit           : None
python           : 3.6.6.final.0
python-bits      : 64
OS               : Darwin
OS-release       : 18.6.0
machine          : x86_64
processor        : i386
byteorder        : little
LC_ALL           : None
LANG             : en_GB.UTF-8
LOCALE           : en_GB.UTF-8

pandas           : 0.25.1
numpy            : 1.17.2
pytz             : 2019.2
dateutil         : 2.8.0
pip              : 10.0.1
setuptools       : 39.0.1
Cython           : None
pytest           : 5.1.2
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : None
html5lib         : None
pymysql          : None
psycopg2         : 2.8.3 (dt dec pq3 ext lo64)
jinja2           : 2.10.1
IPython          : 7.8.0
pandas_datareader: None
bs4              : None
bottleneck       : None
fastparquet      : None
gcsfs            : None
lxml.etree       : None
matplotlib       : None
numexpr          : None
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : 0.14.1
pytables         : None
s3fs             : None
scipy            : 1.3.1
sqlalchemy       : 1.3.8
tables           : None
xarray           : None
xlrd             : 1.2.0
xlwt             : None
xlsxwriter       : None

</details>
