Consider dropping the pytest-runner test dependency

* rfc3339-validator version: 0.1.4

* Python version: any

* Operating System: any



### Description



The `pytest-runner` package is [deprecated upstream](https://pypi.org/project/pytest-runner/). Its README now advises:



> pytest-runner depends on deprecated features of setuptools and relies on features that break security mechanisms in pip. For example ‘setup_requires’ and ‘tests_require’ bypass `pip --require-hashes`. See also [pypa/setuptools#1684](https://github.com/pypa/setuptools/issues/1684).

> 

> It is recommended that you:

> 

> - Remove `'pytest-runner'` from your `setup_requires`, preferably removing the `setup_requires` option.

> - Remove `'pytest'` and any other testing requirements from `tests_require`, preferably removing the `tests_requires` option.

> - Select a tool to bootstrap and then run tests such as tox.



This came up in a review of a proposed `python-rfc3339-validator` package for Fedora Linux.



### What I Did



- Followed `pytest-runner` upstream’s advice (leaving the already-empty `setup_requires` for now).

- Verified I can still run the tests with `tox`.



I also tried to run the tests with `pip install -r requirements_dev.txt` followed by `python -m pytest` or `pytest`, but this didn’t work on Python 3.10 because of the old pinned versions in `requirements_dev.txt`. However, after `pip install --upgrade pip tox coverage pytest hypothesis strict_rfc3339`, I was able to run the tests this way too.



I will offer these suggested changes as a PR.

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