fix: Improve checks for pidfd availability

[achimnol]

• Use os.pidfd_open() that requires a higher kernel version to avoid
  a missing API error in Linuxn kernel 5.1 to 5.2.

[codecov]

Codecov Report

Merging #51 (6306c39) into main (46152be) will decrease coverage by 0.05%.
The diff coverage is 85.71%.

@@            Coverage Diff             @@
##             main      #51      +/-   ##
==========================================
- Coverage   78.55%   78.49%   -0.06%     
==========================================
  Files          16       16              
  Lines        1329     1330       +1     
==========================================
  Hits         1044     1044              
- Misses        285      286       +1     

Impacted Files	Coverage Δ
src/aiotools/server.py	50.00% <75.00%> (-0.17%)	⬇️
src/aiotools/fork.py	74.48% <100.00%> (ø)

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