Broken on Python 3.7 - access to re._pattern_type in Browser.get_link()

In `robobrowser.RoboBrowser.get_link()`

```
File "/usr/local/lib/python3.7/site-packages/robobrowser/browser.py", line 269, in get_link
self.parsed, _link_ptn, text=text, *args, **kwargs
File "/usr/local/lib/python3.7/site-packages/robobrowser/helpers.py", line 51, in find
soup, name, attrs or {}, recursive, text, 1, **kwargs
File "/usr/local/lib/python3.7/site-packages/robobrowser/helpers.py", line 39, in find_all
if match_text(text, tag):
File "/usr/local/lib/python3.7/site-packages/robobrowser/helpers.py", line 16, in match_text
if isinstance(text, re._pattern_type):
AttributeError: module 're' has no attribute '_pattern_type'
```

See also https://github.com/python/cpython/pull/1646