Pandas TimeDelta instantiation returns different object for same value
```python
pd.Timedelta(3,unit= 'M')
Out[26]: Timedelta('91 days 07:27:18')
pd.Timedelta('3M')
Out[27]: Timedelta('0 days 00:03:00')

```
#### Problem description
I tried instantiating Timedelta object using '3M' which returns Timedelta object with 3 minutes rather than 3Months

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

<details>

INSTALLED VERSIONS
------------------
commit: None
python: 3.6.2.final.0
python-bits: 32
OS: Windows
OS-release: 10
machine: AMD64
processor: Intel64 Family 6 Model 78 Stepping 3, GenuineIntel
byteorder: little
LC_ALL: None
LANG: None
LOCALE: English_United States.1252
pandas: 0.24.1
pytest: None
pip: 10.0.1
setuptools: 39.1.0
Cython: None
numpy: 1.16.1
scipy: 1.2.1
pyarrow: None
xarray: None
IPython: 7.3.0
sphinx: 1.8.4
patsy: 0.5.1
dateutil: 2.8.0
pytz: 2018.9
blosc: None
bottleneck: None
tables: None
numexpr: None
feather: None
matplotlib: 3.0.2
openpyxl: None
xlrd: None

</details>
