Importing pandas changes return value of sys.getsizeof?
#### Code Sample

```
a = "a"
import sys
print(sys.getsizeof(a))
import pandas
print(sys.getsizeof(a))
```
#### Problem description
When I run the above code sample, I get output like this:

```
Python 3.5.5 |Anaconda, Inc.| (default, May 13 2018, 21:12:35) 
[GCC 7.2.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> a = "a"
>>> import sys
>>> print(sys.getsizeof(a))
50
>>> import pandas
>>> print(sys.getsizeof(a))
58
```
It looks like the `import pandas` statement changes something about the built-in strings or `sys.getsizeof` function to make it think single-character strings are 8-bytes longer.

Longer strings (e.g. `"aa"` and `"aaa"` etc) do not appear to be affected - we've tried with strings up to 100 characters long.

I've seen this with a small mix of Python and pandas versions, e.g.
Python 3.4.3/Pandas 0.22.0
Python 3.7.0/Pandas 0.23.1

Running on 64-bit Ubuntu 16.04 LTS with all latest updates.

#### Expected Output
```
sys.getsizeof(a)
```
should return the same value before and after `pandas` is imported.

#### Output of ``pd.show_versions()``
This output is from the same interpreter that I used to get my **Problem description** output above.

<details>
>>> pd.show_versions()

INSTALLED VERSIONS
------------------
commit: None
python: 3.5.5.final.0
python-bits: 64
OS: Linux
OS-release: 4.4.0-130-generic
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_GB.UTF-8
LOCALE: en_GB.UTF-8

pandas: 0.23.1
pytest: None
pip: 10.0.1
setuptools: 39.2.0
Cython: None
numpy: 1.14.5
scipy: None
pyarrow: None
xarray: None
IPython: None
sphinx: None
patsy: None
dateutil: 2.7.3
pytz: 2018.5
blosc: None
bottleneck: None
tables: None
numexpr: None
feather: None
matplotlib: None
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>
