Fix milagro bls version#2

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ChihChengLiang wants to merge 7 commits intomasterfrom

fix-milagro-bls-version

Commits on Apr 21, 2020

Commits on Apr 22, 2020

Drop shims for Python 3.6 support
cburgdorf
committed

Commits on Apr 24, 2020

Upgrade bls
cburgdorf
committed

Commits on Apr 26, 2020