Fix pytest deprecation warning for pytest version greater than 5.4

[finsberg]

Running the test using pytest version 5.4 or newer gives the following deprecation warning from pytest

pytest_tornado/plugin.py:70: 17 tests with warnings
  /Users/henriknf/local/src/pytest-tornado/pytest_tornado/plugin.py:70: PytestDeprecationWarning: direct construction of Function has been deprecated, please use Function.from_parent
    item = pytest.Function(name, parent=collector)

Here is a fix that is also backward compatible with older versions of pytest

[okin]

Since this uses pkg_resources you should add a dependency to setuptools.

[finsberg]

pkg_resources is part of the standard library, is it not?

[finsberg]

I am sorry, pkg_resources is not part the the standard library but a part of setuptools (see e.g https://setuptools.readthedocs.io/en/latest/pkg_resources.html). Don't know if it make sense to add this as a dependency to setuptools if it is part of setuptools?

[okin]

Please excuse if I was being unclear.

Since you are using pkg_resources (which comes with setuptools) in the new code you should add a dependency to setuptools in setup.py of this project.

This makes it clear that it depends on modules provided by setuptools and will aid nyone who wishes to packages this for a distribution of their choice. This clearly isn't a problem if people are only using pip but I wouldn't rely on this.

[finsberg]

I totally get your point. I have added setuptools to the install requirements now.

[vidartf]

Since we already import setuptools in setup.py adding it as a dependency might be a little confusing, but I think it is good for dependency tracking 👍 (the bootstrapping issue would also be fixed if we were to add a pyproject.toml file as specified in PEP 518).