setuptools' tests_require deprecation

[ArsenArsen]

Describe the bug when running pip install -v . in the pytest-flask project directory, setuptools produces a warning:

/tmp/pip-build-env-gy9vaagi/overlay/lib/python3.12/site-packages/setuptools/_distutils/dist.py:261: UserWarning: Unknown distribution option: 'tests_require'
  warnings.warn(msg)

it seems that tests_require was removed - it was deprecated for a while: https://setuptools.pypa.io/en/latest/references/keywords.html#keyword-tests-require

test_require seems to be empty anyway, probably best to just drop it

To Reproduce Steps to reproduce the behavior:

1. pip install -v .

Expected behavior no warning

Screenshots If applicable, add screenshots to help explain your problem.

Environment (please complete the following information):

• OS: [e.g. macOS Catalina]: Gentoo GNU/Linux
• Python Version [e.g. 3.8.5]: 3.12.8
• pytest-flask version [e.g. 0.15.0]: trunk