Add warning about 3.0

[sentrivana]

Description

Add a warning about discontinuing 3.0 to potel-base. Once this is merged, we'll release one more alpha to get the warning out there and that's it.

Also contains some "improvements" to the test setup to make tests pass... including removing some tests -- it's fine, the branch will not be used anymore and stuff is working, it's just that the test setup is out of date with some new releases.

Issues

Reminders

• Please add tests to validate your changes, and lint your code using tox -e linters.
• Add GH Issue ID & Linear ID (if applicable)
• PR title should use conventional commit style (feat:, fix:, ref:, meta:)
• For external contributors: CONTRIBUTING.md, Sentry SDK development docs, Discord community

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.61%. Comparing base (bdca31d) to head (2e0d31b).
⚠️ Report is 1 commits behind head on potel-base.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@              Coverage Diff               @@
##           potel-base    #4956      +/-   ##
==============================================
- Coverage       84.85%   84.61%   -0.25%     
==============================================
  Files             158      158              
  Lines           16093    16095       +2     
  Branches         2564     2564              
==============================================
- Hits            13655    13618      -37     
- Misses           1648     1685      +37     
- Partials          790      792       +2     

Files with missing lines	Coverage Δ
sentry_sdk/_init_implementation.py	100.00% <100.00%> (ø)
sentry_sdk/integrations/threading.py	92.18% <100.00%> (ø)

... and 5 files with indirect coverage changes