python2Packages.prawcore: disable on python2

no longer compatible with python2's urllib

```
  prawcore/exceptions.py:2: in <module>
      from urllib.parse import urlparse
  E   ImportError: No module named parse
  builder for '/nix/store/3l1rqygq58hbwyzn6fqx8x4djbmzgggh-python2.7-prawcore-1.4.0.drv' failed with exit code
```