small fix of extract_sex

[djarecka]

assuming that U or u value can be treated as Unknown

[codecov]

Codecov Report

Merging #549 (3c55839) into master (215a155) will not change coverage.
The diff coverage is 0.00%.

@@           Coverage Diff           @@
##           master     #549   +/-   ##
=======================================
  Coverage   83.44%   83.44%           
=======================================
  Files          62       62           
  Lines        6476     6476           
=======================================
  Hits         5404     5404           
  Misses       1072     1072           

Flag	Coverage Δ
unittests	83.44% <0.00%> (ø)

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Impacted Files	Coverage Δ
dandi/metadata.py	79.17% <0.00%> (ø)

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[yarikoptic]

Great, thank you @djarecka !