test: avoid local times near midnight for test_past_swap_days_buttons()

[jennifer-richards]

Fixes occasional failure in test_past_swap_days_buttons(). That test assumed that, in the meeting time zone, a timeslot one second in the past was on the same day as one 9 minutes in the future. The time zone for the meeting was chosen at random. If the test ran at any time more than 51 minutes past the hour, there was a chance that these two meetings would fall on opposite sides of midnight in the meeting time zone. When this happened, the test failed.

This PR introduces and uses a helper method ietf.utils.timezone.timezone_not_near_midnight() that randomly chooses a time zone where timezone.now() represented in that zone is not between 23:00 and 00:59. This is done with a random guess-and-check using pytz.common_timezones. This can change to zoneinfo.available_timezones() when we drop pytz (or before, though with a small risk that zoneinfo suggests a timezone not known to pytz).

[codecov]

Codecov Report

Merging #4642 (85c3ad0) into main (d34e88d) will decrease coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #4642      +/-   ##
==========================================
- Coverage   88.43%   88.40%   -0.03%     
==========================================
  Files         296      296              
  Lines       39716    39730      +14     
==========================================
+ Hits        35122    35125       +3     
- Misses       4594     4605      +11     

Impacted Files	Coverage Δ
ietf/utils/timezone.py	91.17% <100.00%> (+6.17%)	⬆️
ietf/utils/pipe.py	82.60% <0.00%> (-4.35%)	⬇️
ietf/doc/views_search.py	88.81% <0.00%> (-1.45%)	⬇️
ietf/utils/mail.py	79.62% <0.00%> (-0.47%)	⬇️
ietf/submit/forms.py	82.70% <0.00%> (-0.18%)	⬇️

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