remove usages of deprecated `is_string`

[DetachHead]

SeleniumLibrary/src/SeleniumLibrary/__init__.py

Line 27 in f9e9792

from robot.utils import is_string

SeleniumLibrary/src/SeleniumLibrary/utils/types.py

Line 20 in f9e9792

from robot.utils import is_string, timestr_to_secs

these functions are deprecated as of robotframework 7.3, see https://github.com/robotframework/robotframework/blob/master/doc/releasenotes/rf-7.3.rst#deprecated-utility-functions