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Merge pull request #1 from majunkang/master

Fix a typo in ecdsa.py's "Extended Euclidean algorithm", replace old_s with old_t.

"old_t, t = t, old_s - quotient * t" shoud be "old_t, t = t, old_t - quotient * t"

Spurious pdb.set_trace()

Unused import.