devel/py-virtualenv: Update to 20.26.0

- Add a pre-test stage that adds a [tool.setuptools_scm] section to
  project.toml, so that tests can detect the version.
  Otherwise 5 tests fail with an error similar to

________________________________________________________________________________________________________________________ test_fail_no_traceback ________________________________________________________________________________________________________________________

raise_on_session_done = <function raise_on_session_done.<locals>._func at 0x3f63e7392c10>, tmp_path = PosixPath(/tmp/pytest-of-root/pytest-5/test_fail_no_traceback0), capsys = <_pytest.capture.CaptureFixture object at 0x3f63e5c0aca0>

    def test_fail_no_traceback(raise_on_session_done, tmp_path, capsys):
        raise_on_session_done(ProcessCallFailedError(code=2, out="out\n", err="err\n", cmd=["something"]))
        with pytest.raises(SystemExit) as context:
            run_with_catch([str(tmp_path)])
        assert context.value.code == 2
        out, err = capsys.readouterr()
        assert out == f"subprocess call failed for [{something!r}] with code 2\nout\nSystemExit: 2\n"
>       assert err == "err\n"
E       AssertionError: assert  ... nerrn == errn
E         +                     WARNING  pyproject.toml does not contain a setuptools.py:119
E         +                              tool.setuptools_scm section
E           err

tests/unit/config/test___main__.py:52: AssertionError

See also https://github.com/pypa/setuptools_scm/issues/1011

Release changes:	https://github.com/pypa/virtualenv/releases/tag/20.26.0
Reported by:	Bernát Gábor <notifications@github.com>