fix(http2): Check for h2 existence

[BYK]

The new HTTP2Transport needs httpcore and h2 and we only checked for httpcore. This caused runtime errors and dropping of all events during testing as the test platform had httpcore installed but not h2. This patch adds both as conditions for the new transport implementation. Ideally, when we switch out the old transport, we'd silently check for h2 existence only and set the http2 option accordingly.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 84.28%. Comparing base (02d0934) to head (55d3c2f).
Report is 1 commits behind head on master.

✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3690   +/-   ##
=======================================
  Coverage   84.28%   84.28%           
=======================================
  Files         133      133           
  Lines       14260    14261    +1     
  Branches     2405     2405           
=======================================
+ Hits        12019    12020    +1     
  Misses       1490     1490           
  Partials      751      751           

Files with missing lines	Coverage Δ
sentry_sdk/transport.py	84.09% <100.00%> (+0.04%)	⬆️

... and 2 files with indirect coverage changes

[asottile-sentry]