fix test

[Yosukesan]

Unit test failed. I'm using python provided by Debian's system.
I fixed and going to send a patch as pull request. Can you review it ?

$ cat /etc/os-release  | grep VERSION
VERSION_ID="10"
VERSION="10 (buster)"
VERSION_CODENAME=buster

$ which python3
/usr/bin/python3

$ apt-cache show python3 | grep -i version
Version: 3.7.3-1

Test Result

- yosuke@x250:~/Data/debian/python_xbrl$ pytest-3 
- ============================= test session starts ==============================
- platform linux -- Python 3.7.3, pytest-3.10.1, py-1.7.0, pluggy-0.8.0
- rootdir: /home/yosuke/Data/debian/python_xbrl, inifile:
- collected 15 items                                                             
- 
- python-xbrl/tests/test_parse.py F.FFFFFFFFFFFFF                          [100%]

[Joe Cabrera]

Closed with #54

[Yosukesan]

Thanks