Does not run in python3.7 or python 2.7

```
$ python2 --version
Python 2.7.16+

$ python3 --version
Python 3.7.2+
```

```
$ python3 ./corpuscrawler --language tzh --output output-tzh/
Cache-Hit: http://listen.bible.is/robots.txt
Traceback (most recent call last):
File "./corpuscrawler", line 28, in
sys.exit(corpuscrawler.main.main())
File "/home/fran/source/corpuscrawler/Lib/corpuscrawler/main.py", line 1249, in main
crawls[args.language](crawler)
File "/home/fran/source/corpuscrawler/Lib/corpuscrawler/crawl_tzh.py", line 21, in crawl
crawl_bibleis(crawler, out, bible='TZHSBM')
File "/home/fran/source/corpuscrawler/Lib/corpuscrawler/util.py", line 776, in crawl_bibleis
init = crawler.fetch(firsturl)
File "/home/fran/source/corpuscrawler/Lib/corpuscrawler/util.py", line 136, in fetch
if not self.is_fetch_allowed_by_robots_txt(url):
File "/home/fran/source/corpuscrawler/Lib/corpuscrawler/util.py", line 259, in is_fetch_allowed_by_robots_txt
checker.parse(robots_txt.decode('utf-8'))
AttributeError: 'str' object has no attribute 'decode'

$ python2 ./corpuscrawler --language tzh --output output-tzh/
Traceback (most recent call last):
File "./corpuscrawler", line 24, in
import corpuscrawler.main
File "/home/fran/source/corpuscrawler/Lib/corpuscrawler/main.py", line 20, in
from corpuscrawler import (
File "/home/fran/source/corpuscrawler/Lib/corpuscrawler/crawl_aaz.py", line 16, in
from corpuscrawler.util import crawl_bibleis
File "/home/fran/source/corpuscrawler/Lib/corpuscrawler/util.py", line 18, in
from builtins import open, bytes, chr
ImportError: No module named builtins

```