Handle Path object #157

Currently, files assertions are made on strings. It would be handy to accept pathlib.Path objects as it is what the pytest tmp_path fixture returns :

from assertpy import assert_that

def test_foo(tmp_path):
  f = tmp_path / "bar"
  f.touch()
  
  asset_that(f).is_a_file()

fails with :

TypeError: val is not a path

You need to make it a string :

assert_that(str(f)).is_a_file()