Unable to add Timedelta to a Timestamp Interval 
#### Code Sample, a copy-pastable example if possible

```python
>>> import pandas as pd
>>> year_2017 = pd.Interval(pd.Timestamp('2017-01-01 00:00:00'),
                                              pd.Timestamp('2018-01-01 00:00:00'))
>>> year_2017
Interval('2017-01-01', '2018-01-01', closed='right')

>>> year_2017 + pd.Timedelta(days=7)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unsupported operand type(s) for +: 'pandas._libs.interval.Interval' and 'Timedelta'
```
#### Problem description

From the [pd.Interval documentation](https://pandas.pydata.org/pandas-docs/version/1.0.1/reference/api/pandas.Interval.html):

> You can operate with + and * over an Interval and the operation is applied to each of its bounds, so the result depends on the type of the bound elements

However , we can apply the + operation manually on each bounds:
```python
>>> (year_2017.left + pd.Timedelta(days=7), year_2017.right + pd.Timedelta(days=7) )
(Timestamp('2017-01-08 00:00:00'), Timestamp('2018-01-08 00:00:00'))
```

#### Expected Output

The same as the manual operation on each bounds:

```python
>>> pd.Interval(year_2017.left + pd.Timedelta(days=7), 
                         year_2017.right + pd.Timedelta(days=7) )
Interval('2017-01-08', '2018-01-08', closed='right')
```
#### Output of ``pd.show_versions()``

pandas 1.0.1
<details>

>>> pd.show_versions()

INSTALLED VERSIONS
------------------
commit           : None
python           : 3.7.6.final.0
python-bits      : 64
OS               : Linux
OS-release       : 4.4.0-146-generic
machine          : x86_64
processor        : x86_64
byteorder        : little
LC_ALL           : None
LANG             : fr_FR.UTF-8
LOCALE           : fr_FR.UTF-8

pandas           : 1.0.1
numpy            : 1.18.1
pytz             : 2019.3
dateutil         : 2.8.0
pip              : 19.3.1
setuptools       : 41.4.0
Cython           : None
pytest           : None
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : 4.5.0
html5lib         : None
pymysql          : None
psycopg2         : 2.8.4 (dt dec pq3 ext lo64)
jinja2           : 2.10.3
IPython          : 7.9.0
pandas_datareader: None
bs4              : None
bottleneck       : None
fastparquet      : None
gcsfs            : None
lxml.etree       : 4.5.0
matplotlib       : 3.1.2
numexpr          : None
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : None
pytables         : None
pytest           : None
pyxlsb           : None
s3fs             : None
scipy            : 1.3.1
sqlalchemy       : 1.3.10
tables           : None
tabulate         : None
xarray           : 0.15.0
xlrd             : None
xlwt             : None
xlsxwriter       : None
numba            : 0.46.0

</details>
