pd.Timedelta() cannot process offset alias T while pd.infer_freq() return this format.
#### Code Sample, a copy-pastable example if possible

```python
import pandas as pd
series = pd.Series([1, 2, 3], pd.date_range('201701010000', '201701010030', freq='15min'))
print(pd.infer_freq(series.index))
# Error
pd.Timedelta(pd.infer_freq(series.index))
# While this works
pd.Timedelta(series.index.freq)
```
#### Problem description

pd.Timedelta constructor works with e.g. '15min' but not with '15T',
while pd.infer_freq() returns '15T' and pd.Series.index.freq returns '15min'.
This leads to errors.

#### Expected Output

Same behaviour of pd.Timedelta() for 'min' and 'T' offset alias or deprecation of 'T' as an offset alias.

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

<details>
INSTALLED VERSIONS
------------------
commit: None
python: 3.6.4.final.0
python-bits: 64
OS: Windows
OS-release: 10
machine: AMD64
processor: Intel64 Family 6 Model 142 Stepping 9, GenuineIntel
byteorder: little
LC_ALL: None
LANG: None
LOCALE: None.None
pandas: 0.23.1
pytest: 3.6.2
pip: 10.0.1
setuptools: 39.2.0
Cython: None
numpy: 1.14.5
scipy: 1.1.0
pyarrow: None
xarray: None
IPython: 6.4.0
sphinx: None
patsy: None
dateutil: 2.7.3
pytz: 2018.5
blosc: None
bottleneck: None
tables: None
numexpr: None
feather: None
matplotlib: 2.2.2
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
lxml: None
bs4: None
html5lib: None
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: None
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None

</details>
