Import urllib.request explicitly

[csala]

Inside sdv/demo.py there is a line that imports urllib, which later on is used to call urllib.request.
However, urllib does not have the attribute request unless it has been explicitly exported before by using either from urllib import request or import urllib.request statements.

Because of this, when functions from the sdv.demo sub-package are used before anything else, the code raises an AttributeError (see #224 (comment)).

In order to fix this, we should explicitly import urllib.request instead of just urllib.

See the usage examples in the urllib.request docs and this stackoverflow question for reference.

Steps to reproduce

Inside a fresh virtualenv, open a simple python prompt and try to run the sdv.demo.load_timeseries_demo function.

$ python
Python 3.6.9 (default, Oct  8 2020, 12:12:24) 
[GCC 8.4.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> from sdv.demo import load_timeseries_demo
>>> data = load_timeseries_demo()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/xals/.virtualenvs/sdv-test/lib/python3.6/site-packages/sdv/demo.py", line 322, in load_timeseries_demo
    return load_tabular_demo(dataset_name, table_name, data_path=DATA_PATH, metadata=metadata)
  File "/home/xals/.virtualenvs/sdv-test/lib/python3.6/site-packages/sdv/demo.py", line 252, in load_tabular_demo
    meta, tables = _load_demo_dataset(dataset_name, data_path)
  File "/home/xals/.virtualenvs/sdv-test/lib/python3.6/site-packages/sdv/demo.py", line 158, in _load_demo_dataset
    dataset_path = _get_dataset_path(dataset_name, data_path)
  File "/home/xals/.virtualenvs/sdv-test/lib/python3.6/site-packages/sdv/demo.py", line 116, in _get_dataset_path
    _download(dataset_name, data_path)
  File "/home/xals/.virtualenvs/sdv-test/lib/python3.6/site-packages/sdv/demo.py", line 103, in _download
    response = urllib.request.urlopen(url)
AttributeError: module 'urllib' has no attribute 'request'

Temporary Workaround

In order to prevent the issue, one can simply import urllib.request explicitly before running the sdv.demo code:

$ python
Python 3.6.9 (default, Oct  8 2020, 12:12:24) 
[GCC 8.4.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import urllib.request
>>> from sdv.demo import load_timeseries_demo
>>> data = load_timeseries_demo()
>>>