import warnings for line 282

[flake8](http://flake8.pycqa.org) testing of https://github.com/chiphuyen/sotawhat on Python 3.7.1

$ __flake8 . --count --select=E901,E999,F821,F822,F823 --show-source --statistics__
```
./sotawhat/sotawhat.py:281:13: F821 undefined name 'warnings'
            warnings.warn('On Windows, encoding errors may arise when displaying the data.\n'
            ^
1     F821 undefined name 'warnings'
1
```

sotawhat/sotawhat.py

@@ -3,6 +3,7 @@
 import sys
 import urllib.error
 import urllib.request
+import warnings
 
 import nltk
 from nltk.tokenize import word_tokenize