Timedelta always returning False for equality tests for incompatible types
#### Code Sample, a copy-pastable example if possible

```python
import datetime
import pandas

class CustomClass:

    def __init__(self):
        pass
       
    def __eq__(self, other):
        raise Exception("Custom Class eq")

        
custom = CustomClass()
var1 = datetime.timedelta(seconds=1)
var2 = pandas.Timedelta("1s")    

# Following code raises CustomClass exception
var1 == custom

# Following code returns False, does not call CustomClass.__eq__
var2 == custom
```
#### Problem description

I am trying to implement a version of `pytest.approx` for Timedeltas, which basically returns a class with a custom `__eq__` implementation.

`Timedelta("5s") == AproxTimedelta(...)

However it does not work with pandas Timedelta because it always return False, so the `__eq__` of AproxTimedelta object is never called.

This is not the behavior for python datetime timedelta, as can be seen in the example script.

#### Expected Output

`Exception("Custom Class eq")` raised for both cases

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

<details>

INSTALLED VERSIONS
------------------
commit: None
python: 3.6.3.final.0
python-bits: 64
OS: Windows
OS-release: 10
machine: AMD64
processor: Intel64 Family 6 Model 158 Stepping 9, GenuineIntel
byteorder: little
LC_ALL: None
LANG: None
LOCALE: None.None
pandas: 0.22.0
pytest: 3.5.0
pip: 10.0.0
setuptools: 38.5.1
Cython: 0.28.2
numpy: 1.14.1
scipy: 1.0.0
pyarrow: None
xarray: None
IPython: 6.3.1
sphinx: 1.7.1
patsy: None
dateutil: 2.6.1
pytz: 2017.2
blosc: None
bottleneck: None
tables: None
numexpr: None
feather: None
matplotlib: 2.1.1
openpyxl: 1.7.0
xlrd: 1.1.0
xlwt: None
xlsxwriter: None
lxml: None
bs4: None
html5lib: None
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: 2.10
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None
</details>
