pytest warning fixes

[martinhoyer]

1. Opportunistically use "selectable" entry_points.
   The “selectable” entry points were introduced in importlib_metadata 3.6 and Python 3.10.
   While this could be solved by adding entry_points>=3.6 for python<3.10 as a dependency, python3-importib-metadata el8 package currently only have version 0.23.
   See: DeprecationWarning: SelectableGroups dict interface is deprecated. Use select. python/importlib_metadata#298
   Fixes:

tmt/plugins/__init__.py:116
  /opt/tmt/lib64/python3.11/site-packages/tmt/plugins/__init__.py:116: DeprecationWarning: SelectableGroups dict interface is deprecated. Use select.
    for found in entry_points()[entry_point]:

2. The markings pytest.ini does not seem to be working properly.
   Taking this as an opportunity to move it to pyproject.toml and deleting pytest.ini
   Fixes:

tests/unit/test_beakerlib.py:56
  /root/tmt/tests/unit/test_beakerlib.py:56: PytestUnknownMarkWarning: Unknown pytest.mark.web - is this a typo? 

3. tmpdir was missing explicit convertion to str here:

tests/unit/test_utils.py::test_workdir_env_var
  /root/tmt/tests/unit/test_utils.py:179: PytestWarning: Value of environment variable TMT_WORKDIR_ROOT type should be str, but got local('/tmp/pytest-of-root/pytest-2/test_workdir_env_var0') (type: LocalPath); converted to str implicitly