Defer ctypes-dependent startup imports

[guedou]

This PR proposes a fix to #4951.

[codecov]

Codecov Report

❌ Patch coverage is 61.36364% with 17 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.13%. Comparing base (e2e35c0) to head (80bf242).

Files with missing lines	Patch %	Lines
scapy/arch/common.py	56.25%	7 Missing ⚠️
scapy/layers/bluetooth.py	50.00%	7 Missing ⚠️
scapy/arch/linux/__init__.py	60.00%	2 Missing ⚠️
scapy/supersocket.py	80.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5218      +/-   ##
==========================================
- Coverage   81.14%   81.13%   -0.01%     
==========================================
  Files         393      393              
  Lines       98325    98349      +24     
==========================================
+ Hits        79786    79797      +11     
- Misses      18539    18552      +13     

Files with missing lines	Coverage Δ
scapy/automaton.py	81.45% <100.00%> (+0.05%)	⬆️
scapy/supersocket.py	77.74% <80.00%> (-0.19%)	⬇️
scapy/arch/linux/__init__.py	82.77% <60.00%> (-0.19%)	⬇️
scapy/arch/common.py	67.94% <56.25%> (-4.78%)	⬇️
scapy/layers/bluetooth.py	90.65% <50.00%> (-0.49%)	⬇️

... and 5 files with indirect coverage changes

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