get serialNumber from subject

[busla]

Allows the serialNumber to be extracted and validated from cert Subject.

[codecov-io]

Codecov Report

Merging #118 into master will decrease coverage by 0.1%. The diff coverage is 90.9%.

@@            Coverage Diff             @@
##           master     #118      +/-   ##
==========================================
- Coverage   94.89%   94.78%   -0.11%     
==========================================
  Files           3        3              
  Lines         588      595       +7     
==========================================
+ Hits          558      564       +6     
- Misses         30       31       +1

Impacted Files	Coverage Δ
signxml/__init__.py	94.48% <90.9%> (-0.15%)	:arrow_down:

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[kislyuk]

Thank you for your contribution. Sorry about the delay in handling this PR.

To proceed, this will require tests and the tightening of the comparisons in _get_serial_number(). The in operator is inappropriate in both comparisons, I think.

(Tests are failing for an unrelated reason - I'll fix that in master.)

[ovnicraft]

@busla any news about this ? travis is broken