Importing Pandas when python is started with the -b option generates BytesWarning
#### Code Sample, a copy-pastable example if possible

import pandas as pd

#### Problem description

C:\Users\ldbader>python -b
Python 3.6.1 |Anaconda 4.4.0 (64-bit)| (default, May 11 2017, 13:25:24) [MSC v.1900 64 bit (AMD64)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> import pandas as pd
C:\Users\ldbader\Anaconda3\lib\site-packages\pandas\core\dtypes\common.py:20: BytesWarning: Comparison between bytes and string
  for t in ['O', 'int8', 'uint8', 'int16', 'uint16',
C:\Users\ldbader\Anaconda3\lib\site-packages\pandas\io\packers.py:231: BytesWarning: Comparison between bytes and string
  7: np.dtype('int64'),
>>>

Notice the BytesWarning when pandas is imported in python started with the -b command option.  Apparently, there are two places where a Pandas component attempts to compare a string with bytes.  The bytes should be decoded to ensure valid string comparisons.

#### Expected Output

C:\Users\ldbader>python -b
Python 3.6.1 |Anaconda 4.4.0 (64-bit)| (default, May 11 2017, 13:25:24) [MSC v.1900 64 bit (AMD64)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> import pandas as pd
>>>

No warnings should be reported when using the -b switch.

- Our production system is always run with the -b switch to expose potential defects.
- How can we trust Pandas if it is not comparing bytes to strings correctly?

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

<details>
C:\Users\ldbader>python
Python 3.6.1 |Anaconda 4.4.0 (64-bit)| (default, May 11 2017, 13:25:24) [MSC v.1900 64 bit (AMD64)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> import pandas as pd
>>> pd.show_versions(as_json=False)

INSTALLED VERSIONS
------------------
commit: None
python: 3.6.1.final.0
python-bits: 64
OS: Windows
OS-release: 10
machine: AMD64
processor: Intel64 Family 6 Model 94 Stepping 3, GenuineIntel
byteorder: little
LC_ALL: None
LANG: None
LOCALE: None.None

pandas: 0.20.1
pytest: 3.0.7
pip: 9.0.1
setuptools: 27.2.0
Cython: 0.25.2
numpy: 1.12.1
scipy: 0.19.0
xarray: None
IPython: 5.3.0
sphinx: 1.5.6
patsy: 0.4.1
dateutil: 2.6.0
pytz: 2017.2
blosc: None
bottleneck: 1.2.1
tables: 3.2.2
numexpr: 2.6.2
feather: None
matplotlib: 2.0.2
openpyxl: 2.4.7
xlrd: 1.0.0
xlwt: 1.2.0
xlsxwriter: 0.9.6
lxml: 3.7.3
bs4: 4.6.0
html5lib: 0.999
sqlalchemy: 1.1.9
pymysql: None
psycopg2: None
jinja2: 2.9.6
s3fs: None
pandas_gbq: None
pandas_datareader: None
>>>

</details>
