Fix for Flask 2.2.0 breaking changes

[mikenester]

Fix for the following bug: #2147

The web.py change is obviously the fix, but without the changes to uitl.py, test_index_with_https has errors with python 3.7 & 3.8: https://github.com/mikenester/locust/runs/7642011804?check_suite_focus=true

Traceback (most recent call last):
  File "/home/runner/work/locust/locust/locust/test/test_web.py", line 463, in tearDown
    super().tearDown()
  File "/home/runner/work/locust/locust/locust/test/testcases.py", line 1[88](https://github.com/mikenester/locust/runs/7642011804?check_suite_focus=true#step:9:89), in tearDown
    clear_all_functools_lru_cache()
  File "/home/runner/work/locust/locust/locust/test/util.py", line 88, in clear_all_functools_lru_cache
    wrappers = [a for a in gc.get_objects() if isinstance(a, functools._lru_cache_wrapper)]
  File "/home/runner/work/locust/locust/locust/test/util.py", line 88, in <listcomp>
    wrappers = [a for a in gc.get_objects() if isinstance(a, functools._lru_cache_wrapper)]
ReferenceError: weakly-referenced object no longer exists

With this change, a ResourceWarning appears:

ResourceWarning: unclosed file <_io.TextIOWrapper name='web_test_stats.csv' mode='w' encoding='UTF-8'>
  gc.collect()
ResourceWarning: Enable tracemalloc to get the object allocation traceback
/home/runner/work/locust/locust/locust/test/util.py:[84](https://github.com/mikenester/locust/runs/7642594464?check_suite_focus=true#step:9:85): ResourceWarning: unclosed file <_io.TextIOWrapper name='web_test_stats_history.csv' mode='w' encoding='UTF-8'>
  gc.collect()
ResourceWarning: Enable tracemalloc to get the object allocation traceback
/home/runner/work/locust/locust/locust/test/util.py:84: ResourceWarning: unclosed file <_io.TextIOWrapper name='web_test_failures.csv' mode='w' encoding='UTF-8'>
  gc.collect()
ResourceWarning: Enable tracemalloc to get the object allocation traceback
/home/runner/work/locust/locust/locust/test/util.py:84: ResourceWarning: unclosed file <_io.TextIOWrapper name='web_test_exceptions.csv' mode='w' encoding='UTF-8'>
  gc.collect()
ResourceWarning: Enable tracemalloc to get the object allocation traceback

[cyberw]

Thanks! Just one question: what did cache_timeout=None do, and are we sure we dont need it anymore?

[cyberw]

I will add max_age=0 after merging, because it does the same thing that cache_timeout did

[mikenester]

Thanks! Just one question: what did cache_timeout=None do, and are we sure we don't need it anymore?

I'm not totally sure, but send_file already defaulted it to None. So it wasn't being used, as far as I can tell