Openpyxl Timedelta Behavior Inconsistent in Excel Roundtrip
When round-tripping excel files in tandem with Timedelta data, the values returned by `openpyxl` differ from those returned by `xlwt` and `xlsxwriter`. To illustrate:

```python
In []: df = pd.DataFrame([pd.to_timedelta(1, 'D')])

In []: df.to_excel('some_file.xls') # xlwt
    .: pd.read_excel('some_file.xls')
Out[]: 
   0
0  1

In []: pd.set_option('io.excel.xlsx.writer', 'xlsxwriter')
In []: df.to_excel('some_file.xlsx')
    .: pd.read_excel('some_file.xlsx')
Out[]: 
   0
0  1

In []: pd.set_option('io.excel.xlsx.writer', 'openpyxl')
In []: df.to_excel('some_file.xlsx')
    .: pd.read_excel('some_file.xlsx')
Out[]: 
           0
0 1900-01-01
```

Only the `xlwt` option was being tested previously, but I uncovered this nuance while refactoring tests in #19829.

While the `openpyxl` result maintains date-like metadata I question if it's worth being different from the other engines, especially since that metadata misrepresents the object as a datetime and not necessarily a Timedelta (assuming the latter doesn't exist in Excel). Curious to hear others' thoughts

<details>

INSTALLED VERSIONS
------------------
commit: 92dbc78af9f34af83691d86347fc04d2c543b94c
python: 3.6.4.final.0
python-bits: 64
OS: Darwin
OS-release: 17.4.0
machine: x86_64
processor: i386
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: en_US.UTF-8

pandas: 0.23.0.dev0+383.g92dbc78af
pytest: 3.4.1
pip: 9.0.1
setuptools: 38.5.1
Cython: 0.27.3
numpy: 1.14.1
scipy: 1.0.0
pyarrow: 0.8.0
xarray: 0.10.0
IPython: 6.2.1
sphinx: 1.7.0
patsy: 0.5.0
dateutil: 2.6.1
pytz: 2018.3
blosc: None
bottleneck: 1.2.1
tables: 3.4.2
numexpr: 2.6.4
feather: 0.4.0
matplotlib: 2.1.2
openpyxl: 2.5.0
xlrd: 1.1.0
xlwt: 1.3.0
xlsxwriter: 1.0.2
lxml: 4.1.1
bs4: 4.6.0
html5lib: 1.0.1
sqlalchemy: 1.2.1
pymysql: 0.8.0
psycopg2: None
jinja2: 2.10
s3fs: 0.1.3
fastparquet: 0.1.4
pandas_gbq: None
pandas_datareader: None

</details>

