FIX: missing _windows_libsecp256k1.py CONTEXT_SIGN & CONTEXT_VERIFY

[MementoRC]

The flags for the windows _windows_libsecp256k1.py are missing (oversight in the previous PR)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (dc3cdbd) 85.86% compared to head (9560589) 85.86%.

Additional details and impacted files

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##           master     #139   +/-   ##
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  Coverage   85.86%   85.86%           
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  Files          11       11           
  Lines         573      573           
  Branches       66       66           
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  Hits          492      492           
  Misses         45       45           
  Partials       36       36           

[MementoRC]

@ofek Should we update flags.py to only import the non-deprecated flags, but still keep them in the CFFI since they are still part of the SECP256K1 library?
We could also, just leave it as it is now, or remove them completely - let me know I'll submit the next PR depending upon your preference

[ofek]

I would prefer to completely remove deprecated stuff, and please let me know what is breaking compared to v18 so I can document it.

[MementoRC]

Ok, then, i close this PR and resubmit a clean-up one