Add pylint to the lint options.
Clarify reasoning for where to run pylint.

`pylint` gives some pretty useless errors on the tests directory.

 - It's missing `__init__.py` files on purpose. It's not a package.

 - The function name length doesn't matter, since they're not up at 100
   characters; the longer names are better for the tests as they make more
   sense. They're still relatively short for test names.

 - The functions are all missing docstrings but have comments as to what
   they're testing (which, admittedly, could be amended to docstrings, but
   for now they're staying this way).

 - It can't detect the `pytest.raises` feature, which is a known
   problem with `pylint`.
Removing @property from client.url.

Doesn't really need to be a @property field. This does mean that it can
be overwritten or assigned to now, but that's a different can of worms.
Remove @property from client.headers.

Same reasoning as with the url field.
Phrasing.