Fix conf.exts.load() on Python 3.8

[gpotter2]

• shows proper warnings
• do not crash on invalid extension

[codecov]

Codecov Report

❌ Patch coverage is 0% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.00%. Comparing base (1e7a629) to head (7ffc4a9).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
scapy/config.py	0.00%	7 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4808      +/-   ##
==========================================
- Coverage   81.02%   81.00%   -0.02%     
==========================================
  Files         365      365              
  Lines       89064    89069       +5     
==========================================
- Hits        72161    72152       -9     
- Misses      16903    16917      +14     

Files with missing lines	Coverage Δ
scapy/config.py	80.00% <0.00%> (-0.62%)	⬇️

... and 8 files with indirect coverage changes

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