Set level based on status code for HTTP client breadcrumbs

[sentrivana]

• add logic to maybe_create_breadcrumbs_from_span to set the level of the breadcrumb to warning for the client error range (4xx) and to error for server errors (5xx)
• add functionality to the simple HTTP server that we use in some tests to respond with a specific error code
  • we were (and are) still "using" responses in multiple places, but they're not actually active (the activate decorator is missing) and we're making actual requests outside -- we should clean this up
  • we also can't use responses for stdlib/requests tests since they patch something that we patch
• add httpx, stdlib, requests, aiohttp tests for the new behavior
  • restrict the requests tests to 3.7+ since in 3.6, the span is finished before the HTTP status is set for some reason...

Closes #4000

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.22%. Comparing base (8c25c73) to head (fd6cb87).

✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4004      +/-   ##
==========================================
+ Coverage   80.16%   80.22%   +0.05%     
==========================================
  Files         139      139              
  Lines       15397    15406       +9     
  Branches     2597     2601       +4     
==========================================
+ Hits        12343    12359      +16     
+ Misses       2210     2202       -8     
- Partials      844      845       +1     

Files with missing lines	Coverage Δ
sentry_sdk/tracing_utils.py	84.42% <100.00%> (+0.39%)	⬆️

... and 5 files with indirect coverage changes