More scp_send race condition fixes

[pkittenis]

No description provided.

[codecov]

Codecov Report

Merging #341 (c4d9697) into master (cf29d9d) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #341   +/-   ##
=======================================
  Coverage   99.49%   99.49%           
=======================================
  Files          18       18           
  Lines        1578     1580    +2     
=======================================
+ Hits         1570     1572    +2     
  Misses          8        8           

Impacted Files	Coverage Δ
pssh/clients/native/single.py	100.00% <100.00%> (ø)

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