Fix flaky transport test

[sentrivana]

The test_transport_works test is flaking ~7 our of 10 times after adding brotli, but only on 3.6 with gevent and only in CI.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 84.30%. Comparing base (9ae5820) to head (3707aaf).
Report is 1 commits behind head on master.

✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3666      +/-   ##
==========================================
- Coverage   84.32%   84.30%   -0.02%     
==========================================
  Files         133      133              
  Lines       14226    14227       +1     
  Branches     2398     2398              
==========================================
- Hits        11996    11994       -2     
- Misses       1487     1489       +2     
- Partials      743      744       +1     

Files with missing lines	Coverage Δ
sentry_sdk/_compat.py	94.87% <100.00%> (+0.13%)	⬆️

... and 3 files with indirect coverage changes

[antonpirker]

Looks good. Python 3.6 and brotli is probably a combination that is not that common (according to my gut feeling)