Fix Python 3.12 compatibility in multi-line string scanning

[akaihola]

Partially fixes #495.

Starting from Python 3.12, tokenizer emits an f-string and a triple-quoted f-string as a sequence of tokens starting with FSTRING_START and ending with FSTRING_END, instead of a single STRING token.

Note that as expected, we're still getting the 25 other test failures which will be fixed by #497:

FAILED action/tests/test_main.py::test_creates_virtualenv - ModuleNotFoundError: No module named 'pkg_resources'
FAILED action/tests/test_main.py::test_installs_packages[run_main_env0-expect0] - ModuleNotFoundError: No module named 'pkg_resources'
FAILED action/tests/test_main.py::test_installs_packages[run_main_env1-expect1] - ModuleNotFoundError: No module named 'pkg_resources'
FAILED action/tests/test_main.py::test_installs_packages[run_main_env2-expect2] - ModuleNotFoundError: No module named 'pkg_resources'
FAILED action/tests/test_main.py::test_installs_packages[run_main_env3-expect3] - ModuleNotFoundError: No module named 'pkg_resources'
FAILED action/tests/test_main.py::test_installs_packages[run_main_env4-expect4] - ModuleNotFoundError: No module named 'pkg_resources'
FAILED action/tests/test_main.py::test_installs_packages[run_main_env5-expect5] - ModuleNotFoundError: No module named 'pkg_resources'
FAILED action/tests/test_main.py::test_installs_packages[run_main_env6-expect6] - ModuleNotFoundError: No module named 'pkg_resources'
FAILED action/tests/test_main.py::test_installs_packages[run_main_env7-expect7] - ModuleNotFoundError: No module named 'pkg_resources'
FAILED action/tests/test_main.py::test_wont_install_unknown_packages[foo] - ModuleNotFoundError: No module named 'pkg_resources'
FAILED action/tests/test_main.py::test_wont_install_unknown_packages[  foo  ] - ModuleNotFoundError: No module named 'pkg_resources'
FAILED action/tests/test_main.py::test_wont_install_unknown_packages[foo==2.0,bar] - ModuleNotFoundError: No module named 'pkg_resources'
FAILED action/tests/test_main.py::test_wont_install_unknown_packages[  foo>1.0  ,  bar  ] - ModuleNotFoundError: No module named 'pkg_resources'
FAILED action/tests/test_main.py::test_wont_install_unknown_packages[pylint,foo] - ModuleNotFoundError: No module named 'pkg_resources'
FAILED action/tests/test_main.py::test_runs_darker[env0-expect0] - ModuleNotFoundError: No module named 'pkg_resources'
FAILED action/tests/test_main.py::test_runs_darker[env1-expect1] - ModuleNotFoundError: No module named 'pkg_resources'
FAILED action/tests/test_main.py::test_runs_darker[env2-expect2] - ModuleNotFoundError: No module named 'pkg_resources'
FAILED action/tests/test_main.py::test_runs_darker[env3-expect3] - ModuleNotFoundError: No module named 'pkg_resources'
FAILED action/tests/test_main.py::test_runs_darker[env4-expect4] - ModuleNotFoundError: No module named 'pkg_resources'
FAILED action/tests/test_main.py::test_runs_darker[env5-expect5] - ModuleNotFoundError: No module named 'pkg_resources'
FAILED action/tests/test_main.py::test_runs_darker[env6-expect6] - ModuleNotFoundError: No module named 'pkg_resources'
FAILED action/tests/test_main.py::test_runs_darker[env7-expect7] - ModuleNotFoundError: No module named 'pkg_resources'
FAILED action/tests/test_main.py::test_runs_darker[env8-expect8] - ModuleNotFoundError: No module named 'pkg_resources'
FAILED action/tests/test_main.py::test_error_if_pip_fails - ModuleNotFoundError: No module named 'pkg_resources'
FAILED action/tests/test_main.py::test_exits - ModuleNotFoundError: No module named 'pkg_resources'

[my-tien]

Looks fine, just one question: Why is it necessary to test if token end > token start?

[akaihola]

Looks fine, just one question: Why is it necessary to test if token end > token start?

Because the function is looking for multi-line strings like these:

"""line 1
line 2"""

b'''line 1
line 2
'''

f"""multi-line
{x} f-string"""

Strings which fit on one line have the same line number for token start and end, for example:

"one-line string"
'one-line single-quoted string'
f"one-line {x} f-string"
b"one-line bytestring"