fix: Fix breadcrumb timestamp casting and its tests

[BYK]

These tests were failing for me locally as the timestamps were without tzinfo and all were assumed UTC whereas my local timezone is BST at the moment.

This patch fixes the tests along with faulty/incomplete breadcrumb timestamp parsing logic on py3.7 and py3.8.

[codecov]

Codecov Report

Attention: Patch coverage is 86.66667% with 2 lines in your changes missing coverage. Please review.

Project coverage is 84.49%. Comparing base (25ab10c) to head (9099d97).
Report is 1 commits behind head on master.

✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
sentry_sdk/scope.py	0.00%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3546   +/-   ##
=======================================
  Coverage   84.49%   84.49%           
=======================================
  Files         133      133           
  Lines       13844    13855   +11     
  Branches     2927     2930    +3     
=======================================
+ Hits        11697    11707   +10     
- Misses       1422     1423    +1     
  Partials      725      725           

Files with missing lines	Coverage Δ
sentry_sdk/utils.py	86.44% <100.00%> (+0.17%)	⬆️
sentry_sdk/scope.py	87.91% <0.00%> (-0.14%)	⬇️

... and 2 files with indirect coverage changes

[antonpirker]

Thanks @BYK for the PR. (we had a pr for that: #3537) but yours is nicer, so I will close the other one. :-)

[BYK]

@antonpirker ah, sorry 😅 was working on another patch and these failed. Didn't think of looking for an existing PR.

[szokeasaurusrex]

One of the test_datetime_from_isoformat cases fails on a non-UTC system. Opening issue