Fix unconditional openssl import in DTLS

[VincentVanlaer]

No description provided.

[codecov]

Codecov Report

Merging #2779 (d93f678) into master (e97bcb6) will decrease coverage by 0.01%.
The diff coverage is n/a.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2779      +/-   ##
==========================================
- Coverage   98.94%   98.94%   -0.01%     
==========================================
  Files         113      113              
  Lines       16852    16851       -1     
  Branches     3032     3032              
==========================================
- Hits        16674    16673       -1     
  Misses        123      123              
  Partials       55       55              

Files Changed	Coverage Δ
trio/_dtls.py	96.59% <ø> (-0.01%)	⬇️

[TeamSpen210]

Makes sense, if we're already documenting support for this. Though, we should probably add a test to confirm that importing trio is possible without OpenSSL installed. Perhaps by setting sys.modules["OpenSSL"] = None before importing in a subprocess?

[jakkdl]

Ah, thanks for the PR! Looks like another reason for setting up tox environments for testing (#2699), to check that we don't get hard dependencies on anything in test-requirements. And another nail in the coffin for runtime type-checking #2687